After completing at least a dozen Radiant quests for The Minutemen, Preston Garvey will eventually give you this quest. Note that you also need to have destroyed The Institute while aligned with The Minutemen, and you must have also issued the evacuation order in The Institute before you complete that sabotage mission.
Despite all of those prerequisites, this quest is simple and straightforward. Preston just wants you to hunt down a Courser, who also happens to now work for The Raiders. Travel to the randomly designated quest marker. Once there, take out the Courser. Feel free to eliminate the nearby Raiders as well. Speak to Preston to complete the quest.