These events are similar to the Live events from Horizon 4. Around the top of the hour, every hour, new events start and are depicted on the map as giant pink circles. Anna will also alert you of them, as well as if you are in freeroam a banner on the top of your screen will tell you it is starting and to set a waypoint.
For this achievement, you just have to participate and let the 10-15 minute timer of the event run out if you're the only one in it and you don't have to fully complete round 1.