Adding ShareThis widget to an asp.net page

Question

Hello guys! I am integrating a ShareThis widget in my ASP.NET web page. My problem is that I need to know via JavaScript when an item (let's say Facebook) has been selected. Here is the piece of code that I have used. The widget provides an "onclick" property which enables a callback to be called when the widget is being displayed but not when an item is selected. Can anyone help plzzz?

<script language="javascript" type="text/javascript">
   var thisobj = SHARETHIS.addEntry({
   title:'share this page',
   summary:'Sharing is good for the soul.',
   icon: ''
   }, {button:true,onclick:trackerCallback,offsetLeft:10,offsetTop:-267});

   thisobj.attachButton(document.getElementById("mysharebutton"));
</script>