If you purchased Halo Spartan Assault before February 14th, it probably means two things; you're a big Halo fan and love anything related to the series, and you're going to get a sweet new ride in Forza 5, courtesy of the Halo community at Halo Waypoint.
Last year, around the same time that Forza 5 was released, Halo Waypoint teamed up with Turn 10 to create a beautifully designed Halo Spartan Assault themed 1969 Ford Mustang Boss 302 for a few special people. Whilst it may not be as cool as the Forza warthog that got released in a previous Forza title, it did strengthen the bond between the Forza 5 development team and the Halo community just a little more.
Anyway, the awesome 1969 Ford Mustang Boss 302 has now been made available for anybody that decided to purchase Halo Spartan Assault for Xbox One before February 14th, so if you're in that group, you should now be able to find your awesome Halo inspired muscle car inside of your Message Center on Forza 5, under the gifts section.
Unlike DLC cars and other extra content cars, you will not have to purchase the Spartan themed vehicle, and as soon as you "download" it, it will appear in your garage. I'd have to say the car looks pretty wicked.
Unfortunately, if you purchased Halo Spartan Assault after February 14th, you will not be gifted this car, which kinda sucks, especially if you purchased it on February 15th or something, but at the end of the day, you win some, you lose some. Maybe you could pick up a Halo themed livery in the storefront.