After creating a Pull Request with no reviewer set, the reviewer has to click [Edit] only to be able to assign himself.

A button [Assign to me] (or assign to) could be there, aside of the "Reviewer" label when there is no reviewer. Same as we have on Jira to an unassigned task.

It's frustrating have to edit the Pull Request, and also gives the feeling that the reviewer might be changing something else (description...).