I can assign joystick keys to the Toliss A320 for heading and altitude but they dont work on the flight factor B757? Is there a trick?

I don't have FF757,but their A350, some input you could find in different input category, and set it differently to other add-ons (you can set different profile for different aircraft)

If they didn't provide, as FF350 also not provide all of them, you'll need to use dataref, I'ld use flywithlua to "createinput()", Not sure if there are some script already done by others.

Thanks