For anyone interested, with the last updated version of SPAD, B vars are no longer supported, so the previous code will not work.
I have done the same solution as @jim33152 proposed, using vjoy in spad.next can move the switch ---> after installing vjoy, run it, launch MSFS and in options go to Controls, there you will find your VJoy, then assign the native binding for the Propeller Deice to a joystick button in Vjoy.
Finally in Spad, click on Add Event/button pressed/Advanced/Virtual Joystick and choose your Vjoy (probably it would be vJoy #1), for action choose "Press/release a button" and in "target" choose the same virtual joystick button that you chose in the general control options in MSFS for the Prop Deice switch. That is my workaround to assign this switch to a hardware button. The switch now is moving and the green light is shown.
Cheers.