I was still able to claim this DLC despite not owning the game. This may be grounds for Microsoft to provide you with a free game code. Although they may reject you based on the date you claimed the DLC.
If you played Forza Horizon 4 through Xbox Game Pass and purchased DLC content for it, worry not: by having an active, full-paid (not discounted) Xbox Game Pass subscription on 6/25 you will be eligible to receive a game token if you have purchased any extra content for Forza Horizon 4 through your Xbox Message Center. Codes will start to be delivered in the following days, please bear in mind that it might take a while before you get your token.
Either you own the game or there's some sort of anomaly surrounding your account as I get…
Edit: Oh wait you mean on Microsoft…