Found the solution in a wowhead post: https://www.wowhead.com/news/former-in-game-shop-mount-available-for-free-nvidia-lan-50-event-359333?page=5#p6043292
You have to actively opt-in for “GeForce Rewards” on the account page of your nvidia account:
https://www.nvidia.com/en-gb/account/
As soon as I did the opt-in, I could reload the App and see the code in the redeem tab. So the opt-in seems to work retro-actively! 