I don't see any way to bind the elevator trim switch natively in the sim, and I also can't find a way to bind it with Axis and Ohs. Watching sim events through AaO, it appears pushing the trim switch with a mouse click activates the same sim events as using the trim wheel. The only thing I can find in the Behaviors inspector that differentiates the trim switch is for animating it. Otherwise all I see is that the switch activates K:ELEVATOR_TRIM_SET, which is also activated by the trim wheel.
It's possible there's a way to do it via an RPN script (which you'd also need external software for) but I don't know how.
My coding years are far behind me, so maybe (hopefully) someone else can pop into this thread and tell us I'm wrong.