As explained on the manual, there's nothing special you are supposed to do, the cargo stand loading method will happen automatically if ALL of these are true:
- You are parked on one of the UPS Cargo Stand spots:
AND
- You are using a plane with a forward cargo door
AND
- The plane is correctly configured in GSX to have the main cargo door in its proper place
AND
- You have parked in the precise, exact, position for the 757.
If any of these is not happening, the loading will be the same as in every other parking.