you should try java script for that hope your problem will resolve. i have give a sample please check and modify as per your requirement
the javascript code is as :
<script type="text/javascript">
function openWindowForNavigation() {
// getting the current url with query string
var query = document.location.href;
window.open(query +'&cat='+50);
}
</script>
and your hyperlink is
<asp:HyperLink ID="HyperLink1" runat="server" NavigateUrl="javascript:openWindowForNavigation();">HyperLink