using ajax beforeSend to modify data

Question

Let's say I have an Ajax call from jQuery like this:

$.ajax({
   url: myUrl,
   data: myData,
   type:'post'
});

I would like to be able to add to the myData using

$.ajaxSetup({
   beforeSend: function(call){...}
});

The result should be that all ajax calls (both post and get) is modified so if i get an extra parameter IsAjax=true