Display JavaScript Message after record inserted in ASP.Net

Dear Sir,

how to add popup windows JavaScript while value inserted in table with complaint id.

protected void Button1_Click(object sender, EventArgs e)
{
    string employeeId = lblName.Text.Trim();
    string message = str.Text.Trim();
    string ReadOnly = ReadOnlyTextBox.Text.Trim();
    string COMP_LODGE_DATE = DateTime.Now.ToString("yyy/dd/mm");
    string image = "";
    if (FileUpload1.HasFile)
    {
        string fileName = Path.GetFileName(FileUpload1.PostedFile.FileName);
        if (!Directory.Exists(Server.MapPath("~/Images")))
        {
            Directory.CreateDirectory(Server.MapPath("~/Images"));
        }
        FileUpload1.PostedFile.SaveAs(Server.MapPath("~/Images/") + fileName);
        image = "~/Images/" + fileName;
    }
    string conString = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
    string query = "insert_ittest1";
    using (SqlConnection con = new SqlConnection(conString))
    {
        SqlCommand cmd = new SqlCommand(query);
        cmd.Parameters.AddWithValue("@EMP_ID", employeeId);
        cmd.Parameters.AddWithValue("@COMP_MESSAGE", message);
        cmd.Parameters.AddWithValue("@Comp_type", ReadOnly);
        cmd.Parameters.AddWithValue("@COMP_LODGE_DATE", COMP_LODGE_DATE);
        cmd.Parameters.AddWithValue("@image", image);

        cmd.Parameters.Add("@COMP_ID", SqlDbType.Int).Direction = ParameterDirection.Output;
        cmd.CommandType = CommandType.StoredProcedure;
        cmd.Connection = con;
        try
        {
            con.Open();
            cmd.ExecuteNonQuery();
            string COMP_ID = cmd.Parameters["@COMP_ID"].Value.ToString();
            // string COMP_LODGE_DATE = DateTime.Now.ToString();
            //txtrowid.Text = "" + COMP_ID;
            string Message = "Complaints has been locked successfully.";
            string script = "window.onload = function(){ alert('";
            script += Message;
            script += "');";
            script += "window.location = '";
            script += Request.Url.AbsoluteUri;
            script += "'; }";

            ClientScript.RegisterStartupScript(this.GetType(), "SuccessMessage", script, true);
        }
        catch (Exception ex)
        {
            throw ex;
        }
        finally
        {
            con.Close();
            con.Dispose();
        }

        string comp = txtrowid.Text;
        string Date = LabelD.Text;
        string mob = Label1.Text;
        string empid = lblName.Text;
        clientsms();
    }
}