I am having a problem with my code to toggle a checkbox.

I have one other thought, if the code still isn't working.  

You are setting up the variable 'NewToggle' as type boolean, then building a URL string with it.  It may not make a difference, but you might have to instantiate that variable as a string first.  

So, 

var text NewToggle = If([MD Approval]=false,"true","false");

^^ That may be totally incorrect.  Just a thought.

~Rob