On Steam this is super easy.
Family share with an alt, log into the alt, play the tutorial race.
Go to your main, run any race you want, send the challenge to your alt, log into your alt, accept the challenges, and just quit every race. You'll need 10 wins.
In order for challenges to work on RaceNet with Steam, you have to have 100 or less friends. If you have more than that you can "block" them until you're 100 or less and then it will work.
Servers are closing Nov. 8, 2025, idk if this will be affected.