According to the game's PS5 FAQ page, Forza Horizon 5 will ask players to link their Microsoft accounts when it boots up for the first time on the console. Players already need a PSN account when logging into their console, so a Microsoft account is just another step in the process.

Forza Horizon 5 on PlayStation 5 requires a Microsoft account, the company has confirmed. Confirmation comes from a FAQ posted to the Forza support website: “Yes, in addition to a PSN account you’ll need to link to a Microsoft account in order to play Forza Horizon 5 on PS5. This process begins the first time you start up the game on your console.”

Forza Horizon 5 is releasing on PlayStation 5 and PS5 Pro on April 29 or April 25 through Early Access, but you'll need to hold on to your Microsoft account

Forza Horizon 5' developer Playground Games has confirmed PlayStation users will require a Microsoft account as well as a PlayStation Network account in order to access the title.