This Minutemen mission takes you to the Starlight Drive-In, which is north of Lexington.
This drive-in requires a bit of care given the radiated pool in the middle as well as a couple hidden traps. You'll also need to take out a few enemies.
Once it is safe, you'll need to create a radio beacon in the Workshop found in the small shack. After you create a beacon and a generator, you'll need to attach a wire to get the beacon working. To do this, remain in Workshop mode and look for the "Attach Wire" option when highlighting the generator. After doing so, return to Preston Garvey to report.