Server side javascript confirmation in C#

<script type="text/javascript">
    function CallConfirmBox(passedvalue) {        
        if (confirm("Confirm Proceed Further With " + passedvalue + "?")) {
            //OK – Do your stuff or call any callback method here..
            alert("You pressed OK!");
            return true; //important
        } else {
            //CANCEL – Do your stuff or call any callback method here..
            alert("You pressed Cancel!");
            return false; //important
        }
    }
</script>

<asp:Button ID="btnCall" runat="server" Text="Click Me!" OnClick="btnCall_Click" />

protected void btnCall_Click(object sender, EventArgs e)
{
   //service code here and pass your retrieved value from here..
   string myServiceVal = "some value here..";
   ScriptManager.RegisterStartupScript(this, this.GetType(), "CallConfirmBox",  "CallConfirmBox("'+ myServiceVal +'");", true);
   //your rest update code..
}