When you complete a playlist of races on any online playlist mode you will get to pick the next location of races. Once the votes are done the bonus wheel will spin. At random it may land on "75 G achievement for first place session". If you get this then you have to finish first in the session. That means be the highest point scorer by the end of the session, not just in 1 race.
If you tie for first place then all players that are in the tie will unlock it. I unlocked this with 2 other racers. All of us had 50 points to tie first