While you can spend hours digging through old "keygen" archives, the most reliable "activation code" today is simply grabbing the game during a Steam Sale

If your computer is not connected to the internet, you may need to locate an activation.scb file in your Documents folder and upload it to the official activation page via a connected device.

For issues with older non-Steam keys, you can contact SCS Software Support with proof of purchase, your name, and your email.