Forza Horizon 3

The Air Field Car Meet (Outback), is where you can do drag races on the airstrip, unlocked ~1 Million fans.
Just join a showdown (drag race), with each of the car classes (D, C, B, A, S1, S2), you don't need to win. Class X is NOT required!

In addition to what AussieBigfoot said above...

If you don't have the option to start a showdown, it means there's one currently in progress (press RT and there should be flags next to people's gamertags), wait a while for it to finish and you'll be able to start one afterwards.

If you're waiting more than five minutes, it's fair to say the server has probably crashed, so you'll have to exit and try again.

Also if you didn't know, you can switch cars without leaving the car meet. Press and then "change car".

There is a different kind of drag race from the "Showdown" that counts towards this...

This is where you drive into the parking garage and press Y to join an online free roam session. Having done that, you can drive down to the end of the long straight track where there is a blue icon that looks like a sprint race. This is where the drag race can be started. I did it this way for most of my drag races before I discovered the showdown. I only did showdowns in Class C and B but still got the achievement, having done all the other classes in this type of drag race.

Thanks to Pteppic42 for confirming that player created blueprint drags work to get this achievement as well as the stock ones.

Freeroam - airport drag event - start.
Takes up to a minute (the race itself 30 seconds).
Repeat it 25 times and you're done unlocking also another achievement 👍

You race in that airport drag event in every class of car (D, C, B, A, S2, S1, X).
You can do it as you blueprint that event and make sure it's set on every car eligible with no specific class.

All you need to do is join an Online Freeroam Session and once in, simply start the Drag Race event that’s on the right side of the air strip.

You do not need to wait hours for doing the “Showdown” in the parking garage as many of us thought for years.

Most people are down to randomly join events in Free Roam so you should have no trouble.

I got Class D, C, B, A, S1, and S2 done in under 20 minutes back to back with randoms.

Be sure to set your blueprint to allow all classes.

Quickly teleport back to the event once you load back in so other players don’t start an event. You’re able to teleport back to the event quicker than the session recognizes the race is over yet.