I reached out to GuideToaster on how to unlock this and this is what was said:
Change your settings to authentic difficulty and win a race in a multiplayer lobby (I was host, I choose Monte Carlo- Luceram). Then switch settings to easy difficulty and win a solo race (I choose Mexico Media Luna)... and it should pop !
You can change your settings via the main menu, under gameplay settings. Keep the damage to visual only. Winning a solo race is done through the career mode.