support.forzamotorsport.net/requests/new
Before launch, T10ManteoMax said: “These loyalty rewards will be gifted in game to everyone who has played a previous Forza Motorsport or Forza Horizon game.”
forums.forza.net/t/623716
My gamertag/account should show that I’ve played other Forza titles, so I’m guessing the support staff verified it before gifting me the car today.