[WebMethod()]
        public static List<string> GetCliente(string prefix)
        {
            List<string> clienti = new List<string>();
            using (var dbContext = new Entities())
            {
                var q = dbContext.ENTITY.Where(i => i.X1RSL1.ToUpper().Contains(prefix.ToUpper())).Select(i => i.X1CLIL + " " + i.X1RSL1).Distinct().ToList(); 
                foreach (var cli in q)
                {
                    clienti.Add(cli);
                }
                return clienti;
            }
        }

    <script src="Scripts/jquery-ui-1.12.1.min.js"></script>
    <script type="text/javascript">
   
        $(function () {
          $('#<%=txtbxCliente.ClientID%>').autocomplete({
                        source: function (request, response) {
                            $.ajax({
                                url: "pagina.aspx/GetCliente",
                                data: "{ 'prefix':'" + request.term + "'}",
                                dataType: "json",
                                type: "POST",
                                contentType: "application/json; charset=utf-8",
                                success: function (data) {
                                    response($.map(data.d, function (item) {
                                        return { value: item }
                                    }))
                                },
                                error: function (XMLHttpRequest, textStatus, errorThrown) {
                                    alert(textStatus);
                                }
                            });
                        }
                    });            

    });  
    </script>