Would you rather push a fire button to fire the cannon on a tank vs the full up on the stick? This is a very simple process that basically routes the maximum deflection on the stick potentiometer through a push button.
First thing to do is drill a hole and install the push button into your radio,
Now you have to locate the wires you need to tap coming from the potentiometer
Trace the wire from the stick to where it plugs into the board on the Spektrum radio. You are interested in the red and orange wires. Tap into this wires. In the picture above it shows how I ran two white wires, and tapped into the harness using breadboard pins. These white wires are then routed to the swtich you added.
That is it. Pushing the button has the same effect as full up on the throttle, as it bridges a connection on the potentiometer. Do the case back up, and you are done.