Go into settings (top right on the utility bar, looks like three liens with knobs/levers), then go to "Keyboard", then type in the "Search commands" bar: services. This way, anything with the word "service" in it will show up. There, under "Operation", select "Service the airplane with ground trucks" and put in a new key assignment. Then, in the simulator, press that key assignment and watch the services come to life! Make sure you give them a minute or two, as some vehicles may have to drive to your aircraft from a different location at the airport.