Ooooh I found a really cheeky way of getting this by total accident.
Find a normal lobby with either 15% or 25% race.
Simply pass the starting line to make lap 1 appear and then immediatly pull into the inside hoarding with the breaks applied to halt your vehicle. Leave the controller dormant (dont steer or press anything) and just watch the other racers overtake you 6 or 8 times until every other racer vanishes.
The next thing to do is to accelerate and complete half of the laps given then break and halt one more time until the 'time remaining clock' appears.
THEN BELT IT WITH THE ACCELERATER.
You should notice that you will go from last to 1st and complete the race.
When it prompts Ok just press A.
You may want to mix up your lobbies with this method as i was booted twice.
Took me half hour total for this.
I would suggest doing this ASAP in case it gets patched out.