When you are doing the "Dead End" mission during Aftermath, climb up the stairs to get to the top of the facility.

Keep going until you need to climb through a window to get out of an office.

You'll then come across a metal walkway. Look right and you'll see a Longshot. Take it, and then aim it across and at the end of the large crane you can see in the distance.

Successfully shoot the small figurune sitting on the end of the crane and then a Reaver will come flying in and write "Hi" in the sky.