Use session to show message in ASP.Net

Hi

I have imagebutton in page below is code

     <asp:ImageButton ID="IMGTaeed" runat="server" CssClass="IMgTaeedS" 
                        ImageUrl="~/image/TaeedE.png" onclick="IMGTaeed_Click" OnClientClick="return confirm('Would you like to register؟')" />

and Behind code

  protected void IMGTaeed_Click(object sender, ImageClickEventArgs e)
    {

        string data1 = Request.QueryString["id"].ToString();
        SqlCommand _cmd = new SqlCommand("insertestate1AD", _cn);
        _cmd.CommandType = CommandType.StoredProcedure;
        _cn.Open();
        _cmd.Parameters.AddWithValue("@Transfer", lbltran.Text);
        _cmd.Parameters.AddWithValue("@Type", lbltype.Text);
        _cmd.Parameters.AddWithValue("@id", data1);
        _cmd.ExecuteNonQuery();

        Session["Message"] = true;

        _cn.Close();

     


    }

and page_load event

  if (Session["Message"] != null)
        {

         
            this.ClientScript.RegisterStartupScript(GetType(), "Javascript", "<script>alert('you have successfully registered ')</script>");

            Session["Message"] = null;
        }

as you see here when users click on IMGtaeed Button at first it show below message

"WOULD YOU LIKE TO REGISTER"

when users click on OK It insert data into database and I want after that it show message in messagebox that "You Have Registered Successfully"

so I define Session for this message but it didn't work

when I click on IMGtaee button it insert data in database but id didn't show any message in messagebox...

What should I do?

Best Regards

Neda