On "Dead End" during the Aftermath campaign, climb the stairs to reach the top of the facility. Continue until you have to climb through a window to exit an office. Once on the metal walkway, look to the right to find a Longshot near the railing. Take the Longshot and aim it at end the end of the large crane visible in the distance. Shoot the small figure sitting on the end of the crane to make a flying Reaver write the word "Hi" in the sky.