BSPWM doesn't control your shortcuts. You can assign a shortcut for rofi in sxhkdrc (if you're using sxhkd for managing shortcuts).

These are the keybindings I use for rofi:

super + p        
rofi -show drun
super + r        
rofi -show run

If you want to trigger rofi with the super key alone, then you can add this in your sxhkdrc:

Super_L
rofi -show run / drun

However, I wouldn't recommend doing this if you're using super key in your other shortcuts as well. It'll launch rofi every time you press the super key, even when you're simply holding it down to use a different shortcut (for example: super + Return to launch terminal will launch both rofi and terminal).