As with all the other Minutemen Radiant Recruitment quests, this one is triggered by visiting an unenlisted town, talking to Preston Garvey or by listening to Radio Freedom. The settlement in need will be a random pick.
The settlers will ask that you clean up a nearby location of its Ghouls. It's a straightforward task.
You can either focus on just the designated target(s) or wipe out the entire locale of Ghouls. Whatever you decide, return to the settlement when you're done. Then complete the quest by talking to Preston Garvey.