[This cheat has not been confirmed - please comment below or report as necessary]

Go to Recaro Rush race.

Come in last place, then when the money result comes press these buttons in order: X, Y, A, B and then B, A, Y, X.

You will automatically be in the Festival and you can pick any car in the entire Forza Horizon you want for free.

When you save the game it won't be there the next time though, so it is recommended that you don't save the game - unless you want to win races and continue with missions.