As always, you can do this in a Custom Championship. Just select Latvia in the Location Select menu and set Format to Single Race, the achievement unlocks this way too. Set the Vehicle Class to RX Supercars.
In the Vehicle Select menu, scroll all the way to the Fiesta Rallycross (Mk7)
to change the livery. Nitiss' livery is the 5th
one and has an RMC logo on the hood.
Win the race and the achievement will unlock right after.