Can a Last Will legally specify beneficiaries in an external document? For example, can the Last Will point to a url that shows a list of beneficiaries?
This is a close call, in the example that you suggest, because it won't have been executed with the proper formalities and it isn't clear that the content at a url would be fixed in its language at the time that the Will is executed. Subject to an exception for personal property memorandums (and a more subtle one for powers of appointment in trusts) you can't change the terms of a Will once it is signed except by a Codicil executed with the same formalities.
Certainly, the best practice would be to assume that the answer is no.
There are times when a reference to an external document in a will is allowed (e.g. a reference to real property by address rather than a full legal description found in a recorded deed), but a list of beneficiaries would ordinarily not be allowed unless it was effectively a reference to vital statistics records (e.g. "all children born to or adopted by me.")
In general, references to external documents are not allowed when used to establish the nature of the testator's donative intent (a "testator" is someone who writes a will), unless it "describes the writing sufficiently to permit its identification" and can't be modified after the Will is signed, but can be used to establish general facts about reality.
One exception in Colorado is that a "personal property memorandum" designating who will receive specific items of tangible personal property can be incorporated by reference and does not have to be executed with the same formalities as a will. It isn't clear to me if a url could be a valid personal property memorandum and that issue has never been tested in Colorado.
Colorado's probate laws are based on the Uniform Probate Code and would be substantially identical to any other jurisdiction that adopted the Uniform Probate Code's substantive provisions.
The primary statutes governing this (which aren't necessarily easy to understand without context) are:
Colorado Revised Statutes § 15-11-502. Execution--witnessed or notarized wills--holographic wills
(1) Except as otherwise provided in subsection (2) of this section and in sections 15-11-503, 15-11-506, and 15-11-513, a will shall be:
(a) In writing;
(b) Signed by the testator, or in the testator's name by some other individual in the testator's conscious presence and by the testator's direction; and
(I) Signed by at least two individuals, either prior to or after the testator's death, each of whom signed within a reasonable time after he or she witnessed either the testator's signing of the will as described in paragraph (b) of this subsection (1) or the testator's acknowledgment of that signature or acknowledgment of the will; or
(II) Acknowledged by the testator before a notary public or other individual authorized by law to take acknowledgments.
(2) A will that does not comply with subsection (1) of this section is valid as a holographic will, whether or not witnessed, if the signature and material portions of the document are in the testator's handwriting.
(3) Intent that the document constitute the testator's will can be established by extrinsic evidence, including, for holographic wills, portions of the document that are not in the testator's handwriting.
(4) For purposes of this section, “conscious presence” requires physical proximity to the testator but not necessarily within testator's line of sight.
(5) For purposes of this part 5, “will” does not include a designated beneficiary agreement that is executed pursuant to article 22 of this title.
Colorado Revised Statutes § 15-11-503. Writings intended as wills
(1) Although a document, or writing added upon a document, was not executed in compliance with section 15-11-502, the document or writing is treated as if it had been executed in compliance with that section if the proponent of the document or writing establishes by clear and convincing evidence that the decedent intended the document or writing to constitute:
(a) The decedent's will;
(b) A partial or complete revocation of the will;
(c) An addition to or an alteration of the will; or
(d) A partial or complete revival of the decedent's formerly revoked will or a formerly revoked portion of the will.
(2) Subsection (1) of this section shall apply only if the document is signed or acknowledged by the decedent as his or her will or if it is established by clear and convincing evidence that the decedent erroneously signed a document intended to be the will of the decedent's spouse.
(3) Whether a document or writing is treated under this section as if it had been executed in compliance with section 15-11-502 is a question of law to be decided by the court, in formal proceedings, and is not a question of fact for a jury to decide.
(4) Subsection (1) of this section shall not apply to a designated beneficiary agreement under article 22 of this title.
Colorado Revised Statutes § 15-11-510. Incorporation by reference
A writing in existence when a will is executed may be incorporated by reference if the language of the will manifests this intent and describes the writing sufficiently to permit its identification.
Colorado Revised Statutes § 15-11-511. Testamentary additions to trusts
(1) A will may validly devise property to the trustee of a trust established or to be established (i) during the testator's lifetime by the testator, by the testator and some other person, or by some other person, including a funded or unfunded life insurance trust, although the settlor has reserved any or all rights of ownership of the insurance contracts, or (ii) at the testator's death by the testator's devise to the trustee, if the trust is identified in the testator's will and its terms are set forth in a written instrument, other than a will, executed before, concurrently with, or after the execution of the testator's will or in another individual's will if that other individual has predeceased the testator, regardless of the existence, size, or character of the corpus of the trust. The devise is not invalid because the trust is amendable or revocable, or because the trust was amended after the execution of the will or the testator's death.
(2) Unless the testator's will provides otherwise, property devised to a trust described in subsection (1) of this section is not held under a testamentary trust of the testator, but it becomes a part of the trust to which it is devised, and is administered and disposed of in accordance with the provisions of the governing instrument setting forth the terms of the trust, including any amendments thereto made before or after the testator's death.
(3) A revocation or termination of the trust before the death of the testator causes the devise to lapse, but exhaustion of trust corpus between the time of execution of the testator's will and the testator's death shall not constitute a lapse; a revocation or termination of the trust before the death of the testator shall not cause the devise to lapse, if the testator provides that, in such event, the devise shall constitute a devise to the trustee of the trust identified in the testator's will, and on the terms thereof, as they existed at the time of the execution of testator's will, or as they existed at the time of the revocation or termination of the trust, as the testator's will provides.
Colorado Revised Statutes § 15-11-512. Events of independent significance
A will may dispose of property by reference to acts and events that have significance apart from their effect upon the dispositions made by the will, whether they occur before or after the execution of the will or before or after the testator's death. The execution or revocation of another individual's will is such an event.
Colorado Revised Statutes § 15-11-513. Separate writing or memorandum identifying devise of certain types of tangible personal property
Whether or not the provisions relating to holographic wills apply, a will may refer to a written statement or list to dispose of items of tangible personal property not otherwise specifically disposed of by the will, other than money. To be admissible under this section as evidence of the intended disposition, the writing shall be either in the handwriting of the testator or be signed by the testator and shall describe the items and the devisees with reasonable certainty. The writing may be referred to as one to be in existence at the time of the testator's death; it may be prepared before or after the execution of the will; it may be altered by the testator after its preparation; and it may be a writing that has no significance apart from its effect on the dispositions made by the will.
There is not a statutory definition of a "document" or a "writing" in the Colorado Probate Code.
A will is static. Once made it can only be changed by codicil, however, it is generally recommended that you don’t do this - write a new will revoking all former wills instead.
The last thing your heirs want to worry about when they are (presumably) grieving your death is a hard to interpret will written in a non-conventional way. At the end of that road are lawsuits and estranged families.
In the U.S. a codicil is for small amendments, but you can draft a new will if you choose to make large changes or to change the entire document. https://www.wikihow.com/Update-Your-Will
You can use the URL, but a will is intended to be notarized so you should have a copy of whatever is on the website notarized. That would kind of defeat the purpose of the URL, but it can be done.