The car you want to use is the Tier Two Dodge Challenger SRT8 392 which should be available at the start assuming you use the online pass/have the online version.
Before racing head into 'My Vehicles' and change to the Dodge, then 'Customise Player Livery' Remove the pattern and make the Base colour gloss white.
Then go to Custom race and set the following:
Race Type: Race
Vehicle Tier: Tier Two
Rounds: 1 Round
AI Drivers: Very Easy
Grid Order: Random (Or if you got first place in a previous race you can select "Previous Race Result)
Damage: Visual Only
Flashbacks : Unlimited
Select any track you are good at.
Laps: 2 to give yourself time to get ahead if need be.
Then you just need to win the race, use as many flashbacks as you need
Here's a video