I suspect 300 mm/s will be way too fast for your CNC setup. Try 500 mm/min at 50% power and adjust from there.
You can use the “Frame” feature to do this. Hold shift key when you press the frame button. This will only work if you are connected directly to the laser. You will also need to enable the Fire button for this to work.
See here for more information:
If you want to keep working with gcode instead of connecting directly you could create an outline shape of what you want to burn, set it to a very low power level, and export just that frame as gcode. Run that before you run the actual job.