Confirmation messagebox

Same way that your are doing using JavaScript.

But Register that function from codbehind using
ClientScript.RegisterStartupScript(this.GetType(), "Confirm",
"<script>Confirm('" + Are You Sure ?+ "');</script>");

or if use ajax in your page then ScriptManager instate of ClientScript

<asp:button runat="server" id="Button1" onclick="Button1_Click" style="display:none;"  />
...
<script type="text/javascript">
        function callBack(args)
        {
            document.getElementById("Button1").click();
        }
        function openConfirm()
        {
            radconfirm("Are you sure?", callBack);
        }
</script>