There was a guide for this before, not sure where it went.
You will want to go to "horizon tour" on the map, it is located where the first horizon festival was setup.
You will compete in 3 random race events with a random car class with a team of other people online. The good news is it's against AI and you don't need to rely on your team being good to get this achievement. The AI aren't hard, I beat them every race. (Some people are saying they are hard, make sure you upgrade your cars before the race, it makes them much easier)
Since there is only 3 races in a tour you will need to do another tour, for me it popped after I did 3 races on my first tour then on my second tour it unlocked at the end of the second race, making it 5 races total.
The achievement tracker didn't go up at all for me, so if you don't see it go up just keep trying.
You don't need to come first in the whole event, you just need to beat all the AI (the red team), so if you have a really good teammate who is always first then don't worry, just focus on being first vs the AI.
I was able to beat them with stock cars but if you are struggling then you can, quickly before the event starts, upgrade your car to the maximum the event will allow, that should give you a fairly decent advantage against them.
EDIT: You can track progress of this by pinning an accolade. Press Start>accolades>online>horizon tour, it’s 2 up and 5 across, then pin it. (Thanks Agelessred for the tip)