Since the game was delisted from the Microsoft Store in 2018, you cannot purchase it digitally. To play it today:
Even for the legitimate Xbox versions, digital keys are nearly impossible to find today: Forza Horizon 1 & 2 Online Services Closure - Forza.net Forza Horizon 2 License Key.txt
Files named "License Key.txt" bundled with unofficial game downloads are frequently used to distribute . Since the game was delisted from the Microsoft
Sites offering these files often lead to "survey walls" or require payments for keys that do not exist. Why You Can't Buy Digital Keys Anymore Forza Horizon 2 License Key.txt