This race is one of the final events to unlock and you must be at Rep level 50 and is a night race with a 400 rating and the race takes you around the entire map, the race shows up in a random place on the map.
So play through the story and complete discovery A,B and C ,reach Rep 50 then it will unlock the event.
Win this race to unlock the achievement.
*EDIT* it has been said in the comments a few times by some people that you dont have to complete discovery A,B and C for the event to appear or unlock the achievement, you just have to complete the discovery event, I only gave info on what I done to unlock the discovery event and complete it.