Marko Dragic is a man you will first meet at the beginning of Chapter 4 and he is available always after this point, even after you finish the campaign.
He is located
This mission is in 3 parts.
When he is available he will appear with a grey question mark at this location. He will give you a mission involving a small boat in a lake which you must complete for part 1.
Part 2 of this achievement requires you to meet him again in Dover Hill (located at the very most North Eastern part of the map, directly North of Annesburg) however there are some conditions that need to be met before part 2 can be activated.
-It must be at least 24 in game hours since you completed part 1
-It must be between 9pm and 5am in game time
-An active thunderstorm must be currently happening in game
(When I went there for the first time a thunderstorm started automatically so it’s worth you going to Dover Hill first to see if one starts)
If these are met you can complete part 2 of the mission
The switches order is
Part 3 of the mission is to return back to the same place after 2 days of in game time (you can use a campfire and sleep closeby to skip time) and discover the fate of Marko Dragic.
Thanks to crewdy4923 for pointing out that a mission marker does not appear after you have completed part 2. So just wait 2 days of in game time and go back there, even though there is no marker.