The first thing you will need to do is to unlock Dax by doing one of the first missions in the Drug War. After that, call Dax and request work. You have to get a job called Working remotely where Dax will ask you to go to the code postal Depot. Once you reach there, you have to kill everyone and steal the van.
Take the van to the nearest LS Customs to apply a detonation bomb. After that, Dax will ask you to drive into one of the alas MC’s hideouts and park the van near the oil or gas tank.
After that, move away from this location and open up your phone to detonate the bomb. Then all the cops will come up to you and you need to get into your flying vehicle to leave the area. It will you’ll complete the mission and you will unlock the go postal outfit.