Trying to tap into existing laser pointer switch

Usually under the pushbutton there is a square tact switch with 4 pins.
The pins are connected in pairs, so it is enough to find which two pins you have to short to turn the laser on - usually it will be two pins that are closer together than on adjacent sides.
Once you establish which pins you need to connect to, you have to solder a wire to each, bring them out of the case, and install a momentary-on or on/off switch at the end - whichever option you need.