To unlock Horizon Promo, you must purchase the Sunflower Meadows house. Details for doing this can be found here: Solution for Stay Home in Forza Horizon 4
Horizon Promo is a "side-quest" activity where the goal is to photograph every single unique car model in the entire game. You will earn various rewards for reaching milestones along the way, including achievements for your first car and your 200th unique car.
Once you've unlocked Horizon Promo, the game will introduce you to the activity by asking you to take a picture of your car. The achievement will unlock after you take the picture.
After this, a camera icon will appear above any car model which you haven't photographed yet. To photograph these cars to work toward 200 cars, press
while racing, or press
and navigate to Photo Mode
. You can then photograph the cars with
will allow you to save the photo, while using
will save you about 5-10 seconds of processing time (but won't give you the option of saving the photo). Thanks OctagonQontrol
for the info about skipping the picture processing time with
If you'd already taken a picture of a car before the update went live, this achievement will retroactively unlock when you load the game!