I use two ways. Bought the 6" flexible extension to the tire valves that many use to fill the rear tire in a bagger. Works just fine on the trike shock valve. Also from my bicycling days I have a tire pump that is about six to 8 inches long and it works.
Most tire inflators pin inside the chuck will not reach deep enough to push the valve in the stem.
It is H-D's way to protect the shocks. The amount of air in the shocks is so small that a high pressure air supply will damage the system.
How to fill the shocks proper is in the owners manual by the way.