How to toggle (on/off) potion effect when a player hits a keybind

The simplest way is to make a variable, and when keybind is pressed,:

If [variable] = 0 (default)

 strike the player with lightning

 set variable to 1

Else (if [variable] = 1)

 set variable to 0

Then make a procedure on player tick:

When Variable = 1 (means when the keybind was toggled on)

 Apply potion effect