If I remember correctly, there was no actual place to do this. The "thanks for the first game" (and the 3 gift points that came with it) were given if the new player gave a green thumb to their first opponent.
I can't quite tell if there's a mix-up here or not, but I think it needs pointed out that "first game" means your very first one on BGA, not your first one of each new game you learn. I'm pretty sure of that. (That's an old memory cause I haven't seen it talked about for a while.)
And ollyfish, you are far from playing your first game here.