The actual key to use is the shift key, which has the annoying side effect of prompting me if I want to turn on Sticky Keys. Every time I hit it more than five times. Which I have to because holding down the button does not autofire. Since you're building this in Stencyl, just go into the "Controls" in the dashboard and click on whatever button says "Shift" and hit the "X" key to change the command without changing your programming. For autofire, use a boolean variable. When you process the user is holding down the (now) "x" key, check the boolean. If it's false, set it to 'true', shoot the laser, then tell the program to set it to false again after 1 second or whatever. There are many tutorials that recommend this, just look for "Stencyl shooting tutorial" and you're sure to find it.
 "Shoot em up"){kind=icon}