Message will appear on the Xbox live account that has played Halo 5: Guardians, OR Halo: The Master Chief Collection. MUST own the game to use, but can be redeemed without. Same situation as the Forza 6 DLC code, which was posted earlier. Codes started distributing the week of September 5th.
Code is used for Forza Horizon 3, which comes out September 27th.
"The M12S Warthog CST will be available at launch for Forza Horizon 3, and will be sent as a free download to everyone who has played either Halo 5: Guardians or Halo: The Master Chief Collection on Xbox One. Eligible players will receive codes to download the Warthog via an Xbox Live Message, and codes will begin to be sent out during the week of Sept. 5. In addition, players who have not played an Xbox One Halo game will still have the chance to earn the Warthog via a special #Forzathon event coming to Forza Horizon 3 in October."