Reflection map. Open the car in CM showroom, select car body, txMap is the name of the reflection map. Red channel is specular intensity, green channel is reflection sharpness, blue channel is reflection intensity.
Another way, especially if the reflection map alone isn't enough to achieve the desired look, is to make a material change via CSP. In assettocorsa\extension\config\cars\common there are templates for CSP shader changes.