Well, from my point of view, I'm tempted to say that this is what you get when you want to keep it easy.
TARGET GUI is there only because most users don't want to take the time to learn how to use the Script Editor. And you've just reached the limit where you are about to understand what you can get in return.
What you' re asking would be one line for the Script editor:
MapKey(&Joystick, S4, TEMPO('w'+L_SHIFT , 'w' , 300));
For DCS A-10C which has no parking brake, TARGET can even compensate quite easily by holding the key for you: A short press would toggle 'w' press and release in a sequence.
- One SHORT press would set it on, the next off.
- The LONG press would kick in after a delay of 300ms (meaning it's not the SHORT kind) and would hold 'w' down until you release the switch as if it were an ordinary w key.
MapKey(&Joystick, S4, TEMPO(SEQ(DOWN+'w' , UP+'w') , 'w' , 300));
When you know how to use the script editor, the difficulty becomes to try to get the same result from the GUI. The issue is either a nightmare or plain impossible. Difficulties have simply changed sides.
Maybe you can try so steal the script from the GUI and simply insert one of those lines near the end.
The result would certainly work fine if you give it to the Script Editor.