How to define uri adress for GET method in REST

Question

I had this code, and everything worked well

 [OperationContract]
    [WebInvoke(Method = "GET",
         ResponseFormat = WebMessageFormat.Json,
         RequestFormat = WebMessageFormat.Json,
         BodyStyle = WebMessageBodyStyle.Bare,
         UriTemplate = "Product/{id}?fields={fieldsParam}")]
    ResponseData JSONData(string id, string fieldsParam);

I changed my code into format:

    [OperationContract]
    [WebInvoke(Method = "GET",
        ResponseFormat = WebMessageFormat.Json,
         RequestFormat = WebMessageFormat.Json,
         BodyStyle = WebMessageBodyStyle.Bare,
         UriTemplate = "Product/{id}?fields={fieldsParam}")]
    Message JSONData(RetrievePublishedDataInput input);

Where

 [MessageContract]
    public class RetrievePublishedDataInput
    {
        [MessageBodyMember]
        public string id { get; set; }
        [MessageBodyMember]
        public string fieldsParam { get; set; }
    }

But there is an error. There is no way to use MessageContract with UriTemplate, but how I can specify a service address with request arguments?