The Rockstar Energy Drink Car Pack is now available on XBox Live. You can receive it as part of the season pass or the Ultimate Edition of Forza Horizon 3. More information is available here on the Xbox Wire.
AntDaGamer, Forza Games & Products, Xbox Live, XBOX ONE
Forza Horizon Rockstar Energy Drink Car Pack Now Available

Forza Horizon 3 Rockstar Car Pack Key Art