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 on PS5 will require a Microsoft account, which may limit cross-platform progress and multiplayer access. (Xbox.com) The upcoming release of Forza Horizon 5 on PlayStation 5 is stirring up conversation,

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.