In the game, under Settings, Profile or Support - the exact place depends on which game you are playing.
It is worth writing down now rather than looking for it later. Almost every account question - lost progress, a purchase that did not arrive, a restricted account - can only be looked into with it. Without one, the answer is a request for it, and that is a whole round trip of waiting for nothing.
A screenshot of the screen it is on is enough.
