Go to your throttle programming, and then to the SBK button. Then click the little arrow and select 'New advanced command' and set it up like this (backspace being the reverse key in your E: D bindings):
Then be sure to set the reverse option to hold instead of toggle (in E: D control options), that way you will have reverse when you slide it forwards and go forward again when you slide it back; instead of having to switch it twice as a cheap toggle.