That will change the position of your first waypoint, but I understand that this happens once the group is already moving to its first waypoint. Havent tested it, but probably moving waypoints has no effect if the waypoint being moved is the current waypoint of the group.
If this happens, you might try the following for the activation code of the trigger:
MatSqd1 lockWP true;MatSqd1 move getPos vehicle leader MatSqd1;[MatSqd1,1] setWPPos GetPos AmmoTruck;MatSqd1 lockWP false