Service HistoryG 20

A custom setup here means the "Vehicle Setup", accessible from the 'service area' (so when you load in a event and have the option to start the event).

Anyway, once you have the DLC installed, go to the first competition (AlpineStars Trophy) and select the second race (American RallyX Crown). You need to use a car from the Power and Glory Car Pack (such as the Ford Focus ST Rallycross). Once the race is loaded, go into 'Vehicle Setup', change the options available to your liking (though at least one of them has to be modified for the achievement to pop) and race for the first place (you can change the difficulty to 1 in the 'Difficulty and Assists' menu, if you want a guarantee).