If you own the original Xbox 360 disc, you can simply insert it into your modern Xbox console. It will trigger a digital download of a compatible version.
Finding a "registration code" or a .txt file download for Forza Horizon 1 is often a sign of a potential scam or malware. The Reality of Forza Horizon 1 Registration Codes
If you encounter issues with your registration code: Registration Code Forza Horizon 1 .txt Download
Because the game was never officially released for Windows (it was an Xbox 360 exclusive), there is no native PC "registration code" in the traditional sense.
: In the game's menu, look for an option that says "Activate," "Register," or something similar. If you own the original Xbox 360 disc,
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Forza Horizon 1 was originally released in October 2012 for Xbox 360. Unlike modern PC titles, Forza Horizon 1 was never officially released for Windows as a standalone game. The game was an Xbox 360 exclusive, later made backward compatible with Xbox One and Xbox Series X|S via digital download or original disc. The Reality of Forza Horizon 1 Registration Codes
Stay safe, respect software licensing, and enjoy racing games the right way.