Just a quick issue, my gamertag is squirreljp18 and I have played bot halo MCC and halo 5 since launch, and I started playing forza again. I realized that I did not receive a warthog message code for some reason, or maybe I did and it got deleted from updates. I am trying to track down how to still get this car.
As far as I know the only ways to get it was from the original code sent to you a long long time ago or if you happen to find one on the auction house. You should have got a message from Xbox with the code, but it looks like you already knew that. My best advice would be to scroll up in your message log with Xbox or keep an eye on the auction house.