Go to the same store page where you bought the game, log in, and go through like you're going to purchase it again. You'll get to a screen that'll show you all the games linked to your account. You should see Endwalker there with a red button that says "Show registration code". It SHOULD (operative word) show both the early access and regular codes.

I spent an hour and a half trying to figure this out and it worked for me but I'm not making any promises for anybody else