Under 'Online Events,' you should see a couple of daily events on the left. Sometimes there will be a Wager Event. In a Wager Event, you will bet money on how good you'll finish in the final rankings. To earn the maximum return, you must finish in the top tier in the final results. You'll see the results in the 'Online Events' section after the event has concluded, along with your earnings.
Be aware that, like many other online events, you only have one chance to set a time. Focus on being clean and not making mistakes above all else. Having to retrieve the car from an off-track excursion is costly. Be careful and smooth and you should easily finish in the top tier.
Note: It doesn't matter how much you bet. It can be the maximum or minimum amount, or anywhere in between. Just bet an amount and finish in the top tier, and the achievement is yours.
If there is no Wager Event available, check the next day (and continue to do so until you see one).