使用JavaScript的服务器端是否确认消息框

我在JavaScript中创建了一个隐藏字段，这个隐藏字段用于捕获用户从消息框中选择的值。当从下拉框中进行选择时，我会调用以检查从下拉框选择的ID是否已经在表中，如果在表中只找到ID，则我会调用JavaScript来显示消息框。如果用户选择"是"，则显示一些警报，如果用户选择了"否"，则会显示另一个警报。不知怎的，我很难让它工作，当我从下拉列表中进行选择时，什么都没有发生。请帮忙，因为我已经花了好几天的时间来研究这个。谢谢这是我的代码：

 //javascript code
 <script type="text/javascript">
        function Confirm() {
            var confirm_value = document.createElement("INPUT");
            confirm_value.type = "hidden";
            confirm_value.name = "confirm_value";
            if (confirm("Do you want to delete data?")) {
                confirm_value.value = "Yes";
            } else {
                confirm_value.value = "No";
            }
            document.forms[0].appendChild(confirm_value);
        }
</script>

//aspx文件中的下拉菜单

//code behind
  protected void ddl_SelectedIndexChanged(object sender, EventArgs e)
    {
       string ID = ddlName.SelectedValue;
        using (SqlConnection con = new SqlConnection(strConnString))
        {
            using (SqlCommand cmd = new SqlCommand())
            {
                cmd.Connection = con;
                cmd.CommandType = CommandType.Text;
                cmd.CommandText = "SELECT count(*) from MyTable WHERE ID =@ID";
                cmd.Parameters.AddWithValue("@ID", ID);
                con.Open();
                int result = (int)cmd.ExecuteScalar();
                if (result >= 1)
                {
                    ScriptManager.RegisterStartupScript(this.Page, Page.GetType(), "text", "Confirm()", true);
                    string confirmValue = Request.Form["confirm_value"];
                    if (confirmValue == "Yes")
                {
                    this.Page.ClientScript.RegisterStartupScript(this.GetType(), "alert", "alert('You clicked YES!')", true);
                }
          else
                {
                    this.Page.ClientScript.RegisterStartupScript(this.GetType(), "alert", "alert('You clicked NO!')", true);
                }                 
                }
                con.Close();
            }
        }
    }

ScriptManager.RegisterStartupScript(this.Page, Page.GetType(), "text", "Confirm()", true);
string confirmValue = Request.Form["confirm_value"];

When the page renders, create a form element in JavaScript.
Get the value from the form element.