1. open the editor
2. create a random object (f.e. ammo box) and add following script to the initialization:
this addaction ["Open Virtual Arsenal", {["Open",true] spawn BIS_fnc_arsenal}]
3. preview the mission and walk to the ammo box -> select "Open Virtual Arsenal"
4. open the garage and choose a vehicle
5. if you have bad luck, the vehicle spawns too close or in a building