How do I create a keypress action... so that when the user clicks on a MENU popup item an in game action is generated.. like "request pushback" (SHF P) ? thanks

I too have been looking for this.I don't think it's possible.However, in your popup, you can tell the user to press Shift-P to push back.

You can use the menu prompt to trigger different mission actions based on what the user selects from the menu, but I don't think that is what you want.If you are trying to trigger special key events, then you would have to use SimConnect as a custom action.Or you can use what was suggested and must prompt an existing key event that you want the user to press, but they would have to then execute that key event on their own.