Currently I only find documentation that allows me to change my email / username / phone number, but nothing talks about adding a user attribute as an authentication entity for the login.
Write the new identifier to
signInNames.whatever, exactly like the “change identifier” samples work. They just overwrite an existing value, if one didn’t exist, it just gets created automatically.
You can’t add an identifier to /any/ attribute, it has to go into
signInNames attribute, as that is uniquely constrained.