how to display a loading process image in asp.net functions

<script type="text/javascript">
   Sys.WebForms.PageRequestManager.getInstance().add_beginRequest(BeginRequestHandler);
    function BeginRequestHandler(sender, args)
    {
        document.getElementById('<%= lblmsg.ClientID %>').style.display = 'none';
                    
        var state = document.getElementById('loadingdiv').style.display;
        if (state == 'block') {
            document.getElementById('loadingdiv').style.display = 'none';
        } else {
            document.getElementById('loadingdiv').style.display = 'block';
        }
        args.get_postBackElement().disabled = true;
    }
</script>


 <div id="loadingdiv" style="display:none; margin-left:5.3em">
   <img src="icons/throbber_circle.gif" alt="Loading" />      Please wait...
        </div>
      <asp:Label ID="lblmsg" runat="server" ForeColor="Green"></asp:Label>

how to set a linkbutton as default button in asp.net using javascript

txtPassword.Attributes.Add("onKeyPress", "javascript:if (event.keyCode == 13) __doPostBack('" + btnLogin.UniqueID + "','')");