see there is 2 way to do that :
1) event:condition1:keyboard : ctrl pressed
condition2:every x second
action:player:spawn bullet
but this is kinda buggy so i will say try to use the second one its 100% working perfect ^^
2)event:condition1: keyboard: ctrl pressed
condition2: system: compare two values -> bullet.count <= 0
action:player:spawn bullet
in this one you will do bullet when there is no bullet in the screen so you should destroy the first bullet if you want to shoot again :P so add this to actions
action:system:wait -> 2seconds
action:bullet:destroy