Since the game is no longer available for direct purchase on the Steam store, you must look for authorized alternatives:

Even if you gain access to a shared account, the original owner (or the scammer) can reclaim it at any time using Steam Support, leaving you without your progress.

Finding a "free Steam account with Forza Horizon 4 " is a common search for many racing fans, but it’s critical to understand the risks and realities behind these offers. While the prospect of playing this open-world masterpiece for free is tempting, most "free account" links are scams designed to compromise your personal data or infect your device with malware. The Reality of Free Steam Accounts