I have some email into them, yes. I've been in the FM UI for the last X hours trying different settings, etc. I don't see where / how I can make the @my-custom

I have some email into them, yes. I've been in the FM UI for the last X hours trying different settings, etc. I don't see where / how I can make the @my-custom-domain.com email the default. I don't see where I can set a different PW for that same email. Or do I use the same PW for the @fastmail.com address I used to "open" the account?