How to set up M1?

I used qtvcp to make an interface, and when I tested the G code pause function, I saw M1 (execution pause), and I need to turn on the optional switch ( linuxcnc.org/docs/2.8/html/gcode/m-code . html#mcode:m0-m1), then how should I use the M1 pause switch? Where can I see this settings section?

I'm not sure I completely understand...
In qtvcp, the action button's action: 'Optional_stop' controls the response to M1

Alternately you can use a halui hal pin to toggle the optional stop state, there are these three Halui pins related to it.

halui.program.optional-stop.is-on
halui.program.optional-stop.off
halui.program.optional-stop.on

The first is an output that shows the optional stop state, the other two are inputs for changing the state.