I want to assign a custom domain to a Preview environment for any branch (not just a particular branch). But it looks like I can only assign the domain if I specify a specific branch:
The domain troubleshooting guide can help with most custom domain configuration issues. You might be able to use that guide to solve it before a human is available to help you. Then you can come back here and share the answer for bonus points.
You can also use v0 to narrow down the possibilities.