The NCC series are supplied with this facility as standard and the ignition and reverse work just like this. A speed pot is permanently connected and left set to the desired speed. With other controllers it is not quite as simple - one reason is that they have high pedal lockout which prevents the controller working if a demand speed is present when the controller is switched on. However the 4QD series have an inhibit input can be thus used, but requires a little ingenuity.
Details are in an application note .
This can be done but is a factory modification. Last updated 20th October, 2000.