ajax autolist get ID

i implement Ajax autoList on textbox, but when want insert into database how to get the selected row id. What is the syntax to get the selected row id?

<asp:TextBox ID="txtName" runat="server" CssClass="textbox" autocomplete="on" 
            AutoPostBack="True" ></asp:TextBox><asp:AutoCompleteExtender ID="txtName_AutoCompleteExtender" runat="server" 
            CompletionInterval="0" DelimiterCharacters="" Enabled="True" 
            FirstRowSelected="True" MinimumPrefixLength="1" 
            ServiceMethod="GetCompletionList" ServicePath="../AutoList.asmx" TargetControlID="txtName" 
            EnableCaching="true" ShowOnlyCurrentWordInCompletionListItem="True" CompletionListCssClass="completionList"></asp:AutoCompleteExtender>

 public string[] GetCompletionList(string prefixText, int count)
        {
            DataSet ds = new DataSet();
            DataTable dt = new DataTable();
            try
            {
                using (conn)
                {
                    MySqlCommand SqlComm = new MySqlCommand("Select * from customer Where CustName like @param", conn);
                    SqlComm.Parameters.AddWithValue("@param", "%" + prefixText + "%");
                    conn.Open();
                    SqlComm.ExecuteNonQuery();
                    MySqlDataAdapter da = new MySqlDataAdapter(SqlComm);
                    da.Fill(ds);
                }
            }
            catch
            {
            }
            finally
            {
                conn.Close();
            }

            dt = ds.Tables[0];
            List<string> txtItems = new List<string>();
            String dvValues;

            foreach (DataRow row in dt.Rows)
            {
                dvValues = row["CustName"].ToString();
                dvValues = dvValues.ToLower();
                txtItems.Add(dvValues);
            }
            return txtItems.ToArray();
        }