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.

0 Answer
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.
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