Assuming you have the current GSX update ( which supports the 737 NGXu natively ), you simply follow the procedure in the manual for airplanes with a custom fuel system.
- Call the fuel truck
- Wait for the message "use the airplane fuel system" to come up. Now you can change the Fuel quantity from the PMDG FMC.
It's entirely normal you'll see a smaller truck with no crew animation, because the Fuel Hydrant with the elevated platform doesn't support airplanes with refueling points under the wing lower than 3.40 meters. It might come with a future update, since it will require a completely different animation.
So, the smaller fuel truck will just stay there, with its Fuel counter showing the progress, then it will go away.