Currently, Lightburn does not support firing the laser without moving (i.e. make a pin hole). As a workaround you can make a very small circle (like 0.01 mm diameter) on a layer with a slow speed and high number of passes. The laser will functionally not move. You will have to play with the exact diameter of the circle, as once it is below a certain diameter Lightburn will ignore it.
I had seen questions about this on this forum before but didn’t see any answers.
This should fire the laser at 100% without movement. Turn off with G0
Edit: maybe some more information: the value behind the S defines the power level. On most (grbl-) lasers the range is from 0-1000 (really old ones 0-255). So M3 S1000 is full power, whereas M3 S650 is 65%. So you can adjust to your needs.