The online adventure must be completed from event 1 to 4 with the Nissan GT-R Black Edition Horizon Edition. On the 'adventure complete' screen, the achievement will unlock. You can't unlock it if you joined after the first event so stick around and vote for a new championship in that case.
There is another challenge that will reward you with the car in the same #Forzathon set or you may already have one in your garage.