how to show popup and saving the data when the browser is closed

protected void Page_UnLoad(object sender, EventArgs e)
    {   
        DateTime d = System.DateTime.Now;
        string cn = "Data Source=hcl;Initial Catalog=mydata;Integrated Security=True";
        SqlConnection con = new SqlConnection(cn);
        SqlCommand cmd = new SqlCommand("insert into mydate values(@d)", con);
        con.Open();
        cmd.CommandType = CommandType.Text;
        cmd.Parameters.Add(new SqlParameter("@d", d));
        // cmd.Parameters.Add(new SqlParameter("@b", b));
        cmd.ExecuteNonQuery();
        con.Close();

    }