It all depends on your laser high voltage power unit, perhaps you could post some details of this item.
My PSU has 0 to 5 Volt analogue input for power control and this, internally, incorporates a simple DAC (digital to analogue converter) so it will also accept a 5 Volt PWM (pulse width modulation) signal (0 to 100%). As Mach3 has an inbuilt PWM spindle speed control this can be used. However it does have some problems of periodic variation which may not show when controlling a spindle motor but does show with laser control.
At this stage I would recommend that you get everything up and running by using manual control of the maximum laser current.
The on / off M10P1 / M11P1 or E1P0 / E1P1 commands are inserted into the GCode by the post-processor used with your CAD/CAM software.
Tweakie.