<script>
        $("#city").autocomplete({
            source: function (request, response) {
                $.ajax({
                    url: "cityJson",
                    type: "POST",
                    dataType: "json",
                    contentType: "application/json; charset=utf-8",
                    data: "{ 'Prefix': '" + request.term + "'}",
                    success: function (data) {
                        response($.map(data, function (item) {
                            return { label: item.Name, value: item.ID };
                        }))

                    }
                })
            },
            messages: {
                noResults: "", results: ""
            }
        });
</script>

 [HttpPost]
        public JsonResult cityJson(string Prefix)
        {
            CRMcontext crmdb = new CRMcontext();
            var cities = crmdb.cities.Where(p => p.cityName.StartsWith(Prefix)).Select(p=> new {ID=p.ID, Name=p.cityName }).ToList();
            return Json(cities);
        }