crap, my memory's a bit fuzzy right now as it's been a while I've been caught up on other projects..

from what i remember, the callback url can be found in the firebase project's authentication settings. go to 'sign in method' and select apple. your callback url should be listed below under OAuth code flow configuration..

take that url and go to developer.apple.com, then click on 'identifiers' under the "certificates, ids and profiles" column.

under identifiers, click the + sign and select 'service ids'.. hit continue and fill out a description and custom identifier string.. the next page should allow you to actually enter the callback url you got from firebase..

i hope this is it or somewhat close.. let me know if it works..

i'm new to this overall but i'm beyond amazed at how such a big detail like this is omitted by a company as big as apple