Mail v 16 add email account with username and password not email address

Have had an old email address for more than 20 years <******@comcast.net>

Sometime in the past it was necessary to change my username on my Xfinity account to one that was different from my old Comcast email address. The new username is <****@mac.com>

I made the mistake of removing the imap account for this email address when it stopped functioning and now I cannot find a way to add it back. Webmail for the account works fine, but I would prefer to have all my accounts/email addresses in my Mail v16 application.

How am I to manually input a new email account in Mail v16 under MacOS 12.7.2 under these conditions?

The username is indeed an email address, but it is a different domain (mac.com) instead of the correct one (comcast.net) The interface for adding email accounts gives only one field for email address and one for password. I imagine there are terminal commands that I could use to generate a username and password that is different from the email address, but I do not know how to do this.