After unlocking the Dodge Challenger SRT8 392 (tier Two cars), go to your garage and edit the car's visuals. Remove the pattern and set the color to white. Sponsors will not matter. Then, start a custom event with the following settings.

Race Type: Race
Vehicle Tier: Tier Two
Rounds: 1 Round
AI Drivers: Easy

Advanced Options
Damage: Visual Only
Flashbacks: 5 or unlimited

Track Selection
Location: Indianapolis or your choice
Track: Sport Circuit
Laps: 1 or more if needed to overtake opponents.

Select the Dodge Challenger SRT8 392 and finish in first.