Best way to link firebase account indirectly with after logging into an EPIC account from the build?

[Ermelious]

So I'm trying to have the player's epic account (after logging into the EPIC account), login to a 3rd party database, in this situation, Firebase.

What would be the best approach to this?
I'm aware right now that if we click on the login button in a build of the samples in playeveryware that, it will launch the EOS Login Portal in a web browser and will return to the game after Login is completed. Is there a way to have that EOS Login Web Portal, send the user credentials directly over to Firebase to create/link automatically to a Firebase account? I'm guessing it should be credentials like the epic account's registered email, productuserID and possibly the password (or maybe there won't be access to these)?

What can we use from the EPIC account to create and associate with a Firebase account (since the Firebase account is logged in through the EPIC account and not directly by the player)?

I'm assuming doing the above via a callback in the client side build, would be insecure since the player might be able to modify the credentials before sending it off to Firebase via REST Api from the client/unity.

What would be a good approach to implement this? Thank you!