There are multiple ways to do this, but I recommend one of the following:
1) With a friend in a private session
2) In a showdown at the airport car meet
3)
Yin confirms that playground games also count
I did the latter- typically there are not more than 3 people in the showdown race and it takes under a minute to complete.
Go to the airport car meet in the main (not Blizzard Mountain) map. It's the "P" icon. Use
to join a showdown. If the option does not appear, a race is in progress and you need to wait. Typically, only a couple people join so all you need to do is finish. If you are having difficulty, use a muscle car like the '68 Dart HEMI upgraded to A class as lower class cars get a headstart.
If you have not unlocked any other challenges yet this Forzathon, you will also unlock