I just upgraded (again) yesterday to the latest V1 Marlin release. I tested the Laser menu items just now, and the results are inconsistent. Sometimes the menu didn’t work, sometimes it worked, more often than not, it flashes the laser on and then goes off. There was a rewrite of the laser code incorporated in the latest V1 Marlin release, and apparently the Laser menu is buggy. As part of this new code, a new laser safety timeout was added, which makes my next suggestion more complex.
You can write g-code to turn the laser on for a specified amount of time and save it on the SD card. You would then have to run that file to turn the laser on. See this topic for the code Kyle found to work around the safety timeout.