The car you see above is the Chryslus Rocket '69. The fusion-powered single-seater has been a fixture of the Fallout series, but those cars generally look something more like the second picture in the gallery above (taken from your author's copy of the game – gotta take advantage of those chances to play video games during the work day, right?). The car that's being offered to Forza Motorsport 6 and Fallout 4 players is more pre-apocalyptic than post, meaning it's free of 200 years of rust and nuclear fallout. In fact, as with so many of the vehicles in the Fallout universe, it looks like it'd be right at home on the stage of a 1950s auto show.
A code to download your free Rocket '69 should pop up in your Xbox One inbox at some point over the next seven days – it's not in mine as of this writing, for example. And if you haven't played Forza 6 or Fallout 4, Turn 10 will offer the code to anyone that picks up a copy of either game between April 14 and April 30, with codes slated to arrive in early May.