Need code to confirm and proceed in code behind page

protected void btnsubmit_Click(object sender, EventArgs e)
        {
            // do some code and get the value
            bool value = true;
            if (value)
            {
                string script = " <script type=\"text/javascript\"> var val = confirm('ur message ???');  if(val){calfun();}  </script> ";
                //  this.Page.ClientScript.RegisterStartupScript(typeof(Page), "alert", script);
                //ScriptManager.RegisterStartupScript(this,typeof(Page), "alert", message);
                ScriptManager.RegisterStartupScript(this, typeof(Page), "alert", script, false);
            }
        }

<script type="text/javascript">
       function calfun()
       {
           alert('calling');
// to call a server function.. ( create a button with style as dispay:none  , and call that button click event  )
       }

   </script>