I pre ordered Cyberpunk 2077 (Windows version) and it mentions a 30% discount code which should be somewhere in my GOG library under "More --> Serial Keys", however: I can't even find the "More" button or option or whatever it should be there. I only see a couple of shelves with my games, I can click an arrow key on the game itself, but that does not lead to anything looking like a serial keys option. I feel completely lost and dumb now. Please tell me where it is.

Ok, I found it myself.

You have to go to GOG.COM on your browser and login there. Then go to your account --> games.
Select the down arrow on the Cyberpunk 2077 game, select "View Downloads". Then in the new Cyberpunk screen that pops up underneath select More --> Serial Keys.