Detect Shift key + click event on iCheck checkbox

My web page uses iCheck checkboxes. I am trying to get event when Shift key is pressed along with clicking on checkboxes. but no where in the documentation, iCheck gives that notification like normal document.click(function()) gives.

I am using

$('input[name=selectinp]').on('ifChanged', function(event){ 
...
});

Here, event.shiftkey is undefined. Please help.