It was just a guess. I think someone in the XBL deals thread said that it was from that. I just assumed it was from that.
I do also own Halo MCC, so maybe it's because of that. I don't own Halo 5 and I haven't pre-ordered any games.
The Xbox message says...
Thanks for being a loyal Halo fan! Redeem this code to unlock Halo’s iconic Warthog in Forza Horizon 3, releasing September 27th.
Update - Found some more info about it.
http://www.forzamotorsport.net/en-us/news/fh3_gonegold
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 September 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.