Stadia is Google’s gaming platform that lets you instantly play your favorite video games on screens you already own. Access to Stadia is free – you only pay for the games. Stream games directly to your favorite compatible devices. They’re ready wherever you have internet, without waiting for installs, downloads, or updates.
If the "Claim" button doesn't automatically give you the Stadia credit, look for a code in the URL (example - ?invite_code=FT7CU7FFHYTKD48N). You can apply this code in your Stadia account settings on desktop.
Promotion open until available claims expire. See comments for details.