To get a signed user ID, you need to enable Account LinkingAoG in your application.
In the AoG settings, enable Account Linkingwith the authorization code as the type of grant. Fill out your customer information and select a scope.
Now, when the user uses your application, he will ask you to log in, and on your AoG server you can get it authorization code.
Now, using this authorization code, you can get the required user ID and use it for the firebase administrator.
You can find additional information about the Account link here .
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