Submit form when Item is selected from jQuery Autocomplete plugin in ASP.Net

The article you are referring uses old Autocomplete plugin that does not have select event

Refer the following

Implement jQuery Autocomplete using Web Service in ASP.Net

Here you can add select event and submit form

    <script type="text/javascript">
    $(document).ready(function () {
        $("#<%=txtSearch.ClientID %>").autocomplete({
            source: function (request, response) {
               $.ajax({
                   url: '<%=ResolveUrl("~/Service.asmx/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) {
                        alert(response.responseText);
                    },
                    failure: function (response) {
                        alert(response.responseText);
                    }
                });
            },
            select: function (e, i) {
                $("#<%=hfCustomerId.ClientID %>").val(i.item.val);
                document.forms[0].submit();
            },
            minLength: 1
        });
    }); 

    </script>