|Cargobob is one of two setup missions required for The Merryweather Heist (Offshore), with the other one being Minisub.|
Play as Trevor until you get a text from Wade saying the only chopper that is suitable is located at the Fort Zancudo base, which is marked as "HS" on your map. As soon as you enter the base, you'll get a four-star wanted level. Locate the chopper in the base, which is the blue dot and take off with it.
As soon as you take off, you'll get a warning to lose the military helicopter that will now be pursuing you. Head away from it and keep close to the ground, trying to tip the chopper to gain speed. Once you do lose the military chopper, all your stars will be gone, so fly it back to Trevor's airstrip to finish the mission.