It has been proposed to make a region of northern California and southern Oregon into a separate state called Jefferson. I think a number of counties in northern California have supported it in referendums (or referenda, if you like) and the legislature of California has expressed a willingness to consider it if enough counties support it. Under the Constitution of the United States, Article IV, Section 3, Clause 1, the consent of the legislature of California would be needed and Congress would need to decide to admit the new state.
And likewise that of Oregon, except that no action has been taken there, and interest seems to be lacking.
So let us suppose part of California ultimately becomes the state of Jefferson, and the new state is admitted with a state constitution that explicitly names counties in Oregon that may join whenever they so choose, provided of course that Congress and the legislature of Oregon approve the measure, without any further action by the legislature of Jefferson than that clause in the state constitution.
For the purposes of Article IV, Section 3, Clause 1 of the U.S. Constitution, would that provision in the state constitution of Jefferson be sufficient legislative consent from the state of Jefferson, so that no act of the ordinary legislature of Jefferson would be needed?