@kresol You just treat the laser as a CNC spindle. The S value (usually from 0-255 but the Eleks firmware uses 0-1000) controls the power, after you turn it on with the M3 command.
This code will draw a small 10 mm box at full power using default firmware - change the S255 to S1000 for Eleks firmware.
X10 Y10 M03 S255 F1000
X10 Y0 M03 S255 F1000
X0 Y0 M03 S255 F1000
X0 Y10 M03 S255 F1000