<script>
        function PopUp() {
        debugger;
        alert('TEST');
        }
     </script>

<asp:ScriptManager ID="ScriptManager1" runat="server">
 </asp:ScriptManager>

//Add the script  after <form> tag
   bool isClientScriptBlockRegistered = ClientScript.IsClientScriptBlockRegistered("ShowStatus");
   if (!isClientScriptBlockRegistered)
   {
   ScriptManager.RegisterClientScriptBlock(this.Page, this.GetType(), "ShowStatus", "PopUp();", true);
   }