Rainer, in the "switch assignment" menu there is a "toggle" sub header, just go into that and you can set toggle to ON or OFF for any switch.
The switch/pushbutton will be found by the Tx on installation, you'll just need to find out what assignment the Tx gives it. Just go into the control/switch assignment menu, enter one of the controls/switch, and push the button and make a note of the number, then change it's function in the toggle menu.