How to replace 'machine on' GUI button with a physical button

On my control panel, I have a "Start" button that I'd like to use as a mechanical equivalent to pressing the "Machine On" GUI button. Let me elaborate. When I press the "Start" button, it should act as though I have pressed the "machine on" GUI button with my mouse.

I've poked around the pins in HAL and I can't quite get the right one. Machine-is-on toggles acts as an output that toggles when I press the GUI button with the mouse, but that is different from what I want to do. I'm looking for the input to all of that process so I can tie my physical button to the input. Does that make sense?

The Halui hal pin "halui.machine.on" does exactly what it appears you are describing.
linuxcnc.org/docs/html/man/man1/halui.1.html

To use it connect that hal pin to an input triggered by a momentary switch or button.

you may connect this button as you are on a parport like
MASHINE HAL
net external-mashine-on <= parport.0.pin-12-in

postgui hal
net external-mashine-on => halui.machine.on


postgui shoudt be used as it is after the GUI is loaded and about 1000times a second called