Actually keys are handled as defsult purchases. Please follow this guide to get you key: https://jni.itch.io/ancient-warfare-3/devlog/18776/steam-release-update-15
I don't have "claim steam key" button.
Can you can write me an email (janniknickel.info@gmailcom) with your key, so that I can check out what's wrong?
I'm having that same problem! Have you found a solution?