I'm not sure what race 'unlocked' it for me, but I suspect either the Clio Cup, or M1 Procar championships (Because those were the two difficult championships of the season?). I got to do my first Le Mans race as the last race for Season 3. I shortened it to 2 hours of course
You'll get the Le Mans invitation by achieving 50 podium finishes on your career. That includes main series that you are in and the invitationals that you've been doing.