That's not a custom domain
If you DM @brugeman I think he can set you up with one if you use npub.pro He wanted to test it out.
Can’t you just CNAME to that?
only if npub.prob supports it
We do now. There's Custom domain section in settings, released to public access as you are asking. You can also self host on GitHub https://blog.npub.pro/post/how-to-self-host-on-github/