Pass Dictioanry object into WCF using JSONP

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WCF

I have problem to call WCF function pass dictionary object (key, value ) as parameter using JSONP(for cross domain) and get back string value.

string PostCustomDataJSONP(Dictionary<string,> Fields);

Please help me, what code i use in interface attribute and in jquery?

interface......

[OperationContract]
       [WebGet(BodyStyle = WebMessageBodyStyle.Bare,
       RequestFormat = WebMessageFormat.Json,
       ResponseFormat = WebMessageFormat.Json,
       UriTemplate = "/PostCustomDataJSONP/")]
       string PostCustomDataJSONP(Dictionary<string,> Fields);

function for testing ...

public string PostCustomDataJSONP(Dictionary<String, Object> Fields)
       {

           foreach (var i in Fields)
           {
               var key = i.Key;
               var val = i.Value;
              // test the data here....


           }

           return "12345";
       }

jqury ......

$(document).ready(function () {
       var wcfServiceUrl = "http://localhost:13561/Service.svc/";
       // sets both dropdown lists empty

       alert(wcfServiceUrl);
       // JSONP CALL TO WCF SERVICE
       $.ajax({
           cache: false,
           url: wcfServiceUrl + "PostCustomDataJSONP",
           // data: "{}",
           data: "{'Fields': {'FirstName':'" + 'shabber' + "'," + "'LastName':'" + 'raza' + "'}}",

           jsonpCallback: "entity",
           contentType: "application/javascript",
           dataType: "jsonp",
           error: function () {
               alert("list failed!");
           },
           success: function (list) {
               alert('hi1');
               //  alert(list);
               });
           }
       });