To just left click, put LButton in the 'Fire Sequence' box. This is designed for games, but should work for any application where you need to click at a specified rate while you hold a button.
I implemented a rudimentary version in a similar app I wrote called Fire Control. Implementing user-definable hotkeys is a bit of a problem in AHK as the Hotkey GuiControl does not support mouse or joystick.
