You are required to complete a race in the 2016 Ferrari FXX K at Mugello
You are not required to win the race, just complete it.
To start off head to Single player > Freeplay.
Hit RB to go over to setup.
Select Car: (If you do not own the vehicle simply go to the rent option)
2014 Ferrari FXX K*
*as pointed out by Fish02 the car is actually a 2014 vehicle.
However the achievement states 2016, the 2014 car still unlocks the achievement.
Race Setup:-
Environment: Mugello Autodromo Internazionale
Track: Club Circuit (or full if you would prefer)
Laps: 1
Max Drivitars: 23 (default)
Scenario: Day
Car Division: Any
**Set the scenario to rain and you will also unlock this achievement as soon as the race starts (if you haven't already unlocked it)
Complete the race and the achievement is yours.