https://xenonfossil.itch.io/creature-creation-station/devlog/1454665/steam-keys-...
Here's the devlog with the instructions! You'll have to type in the itch link provided in the tiny envelope in your physical case first, to attach the game to your itch account, in order to be eligible for claiming the Steam Key. Let me know if you're still having trouble though! I think a few of the early itch codes might be glitching when you try to get an external key.