<div>
    Enter search term:
    <asp:TextBox ID="txtSearch" runat="server" />
    <asp:HiddenField ID="hfCustomerId" runat="server" />
    <asp:Button ID="Button6" Text="Submit" runat="server" OnClick="Submit" />
</div>

<script type="text/javascript">
    $(function () {
        $("[id$=txtSearch]").autocomplete({
            source: function (request, response) {
                $.ajax({
                    url: '<%=ResolveUrl("~/DesktopModules/DNN_Ajax/Services/AutoComplete.aspx/GetCustomers") %>',
                        data: "{ 'prefix': '" + request.term + "'}",
                        dataType: "json",
                        type: "POST",
                        contentType: "application/json; charset=utf-8",
                        success: function (data) {
                            response($.map(data.d, function (item) {
                                return {
                                    label: item.split('-')[0],
                                    val: item.split('-')[1]
                                }
                            }))
                        },
                        error: function (response) {
                             response.responseText);
                        },
                        failure: function (response) {
                             response.responseText);
                        }
                    });
                },
                select: function (e, i) {
                    $("[id$=hfCustomerId]").val(i.item.val);
                },
                minLength: 1
            });
        });
</script>

protected void Submit(object sender, EventArgs e)
{
    string customerName = Request.Form[txtSearch.UniqueID];
    string customerId = Request.Form[hfCustomerId.UniqueID];
    ScriptManager.RegisterStartupScript(Page, this.GetType(), "alert", " ame: " + customerName + "\\nID: " + customerId + "');", true);
}