I am using a web method an an ASPX page to return the data (a List<object> collection) but don't know what I need to do to specify the 'geonames' part, or whatever would be relevant for my application.

My c# code is as follow:

2.         [WebMethod]
3.         [ScriptMethod(ResponseFormat = ResponseFormat.Json)]
4.         public List<TheEntity> SearchEntitesJSON(string title)
5.         {
6.             Manager mng = new Manager();
7.             List<TheEntity> entList = mng.SearchEntites(title);
8.             return entList;
9.         }

My jquery in the markup is virutally the same as in the autocomplete JSON datasource example except for I have replaced the url with my method above and changed dataType to JSON instead of JSONP. My script is as follows: 

2. <script type="text/javascript">
3.   $(function() {
4.     function log( message ) {
5.       $( "<div>" ).text( message ).prependTo( "#log" );
6.       $( "#log" ).scrollTop( 0 );
7.     }
8. 
   
9.     $( "#city" ).autocomplete({
10.       source: function( request, response ) {
11.         $.ajax({
12.           url: "Post.aspx/SearchEntitesJSON",
13.           dataType: "json",
14.           data: {
15.             featureClass: "P",
16.             style: "full",
17.             maxRows: 12,
18.             name_startsWith: request.term
19.           },
20.           success: function( data ) {
21.             response( $.map( data.entities, function( item ) {
22.               return {
23.                 label: item.Title,
24.                 value: item.Id
25.               }
26.             }));
27.           }
28.         });
29.       },
30.       minlength: 2,
31.       select: function( event, ui ) {
32.         log( ui.item ?
33.           "Selected: " + ui.item.label :
34.           "Nothing selected, input was " + this.value);
35.       },
36.       open: function() {
37.         $( this ).removeClass( "ui-corner-all" ).addClass( "ui-corner-top" );
38.       },
39.       close: function() {
40.         $( this ).removeClass( "ui-corner-top" ).addClass( "ui-corner-all" );
41.       }
42.     });
43.   });
44.     </script>