Because of expiring car and music licenses, Microsoft officially delisted Forza Horizon 2 from the digital Xbox Store in 2018. You can no longer buy it digitally. Your best and safest option is to buy a used physical copy:

The only legitimate way to play the game now is by purchasing a physical disc for the or Go to product viewer dialog for this item. from a second-hand retailer. Better Alternatives for PC Players

Emulators do not use "license key.txt" files. They run the game files directly. If an emulator or a game download asks you for a product key to "activate" the software, it is likely a fake version of the emulator. 4. Better Alternatives for PC Gamers

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Scammers often create "license key" or "activation" files to lure people into downloading malware. Since there is no official PC installer, a license key would have nothing to activate. How People Actually Play on PC

Because of this timeline, no PC port of Forza Horizon 2 was ever coded by Playground Games or Turn 10 Studios. The Legal and Digital Status of the Game

You must possess a legitimate copy of the Xbox 360 game, which you would need to dump (convert to a file) yourself.