>Actually, it seems to be something like navigate('REZ://...

I guess I should admit defeat here (although I may ask my colleague for help).

I've added a hidden input and I've changed the code to be

var form = document.getElementById('frmAddItem');	  
	  
	  document.getElementById('txtFake').value = 'btnRemoveOverdue';
	  //form.action = form.action + '?btnRemoveOverdue=Remove+Overdue'  ;
	  form.submit();

but the passed string now contains an empty txtFake. May be I need to set its text instead of value?

UPDATE. Tried some other things but looks like C++ is not recognizing programmatic changes in DOM. I found a different solution.