Once you've acquired all 10 of the cars you will unlock the last event which is a showcase event against the cargo plane which is located near the garage. Complete the event by finishing ahead of the Cargo plane in the event standings to unlock this, you do not need to beat the AI cars so long as you finish ahead of the Cargo plane.
If needs be you can lower the AI difficulty and don't forget your rewinds from pressing
if you mess up. I also highly suggest turning off assisted braking. The race lasts about 3minutes.
This will be familiar to Horizon 2 players and even easier with nitrous. Save the Nitrous (
) for the final stretch of the runway just incase you need it.
Reabo has made a video of the race for anyone that wants to see ahead of time