HELP! JSON posting of params and JSON string

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JSON

HI

i am trying to post data to a json api.
this is how my code looks. It's returning an error from the server, the guys up there are telling me that i'm sending everything as a json string and that i should post params of which only one parameter is a JSON stirng. ie. campaign_uuid

public class JsonSaleData
    {
        [DataMember]
        public string contact_attributes { get; set; }
        [DataMember]
        public string campaign_uuid = "9d119cce-25ea-46bc-b7bc-cba7e8323e91";
        [DataMember]
        public string user_credentials = "OCPdNbeltviij8C1RLcf";
        [DataMember]
        public string license_id = "1";

    }

public class JsonSaleData
private string PostSaleToClient(string url, JsonSaleData _SaleData)
        {

            
            string strResult = "";
            try
            {
                ASCIIEncoding encoding = new ASCIIEncoding();
               

                System.Net.HttpWebRequest httpWebRequest = (HttpWebRequest)System.Net.WebRequest.Create(url);
                httpWebRequest.Method = "POST";
                httpWebRequest.ContentType = "application/json; charset=utf-8";

       
                DataContractJsonSerializer ser = new DataContractJsonSerializer(typeof(JsonSaleData));
                MemoryStream ms = new MemoryStream();
                ser.WriteObject(ms, _SaleData);
                ms.Position = 0;
                String json = Encoding.UTF8.GetString(ms.ToArray());

                
                StreamWriter streamWriter = new StreamWriter(httpWebRequest.GetRequestStream());
                streamWriter.Write(json);
                streamWriter.Flush();
                streamWriter.Close();

                System.Net.HttpWebResponse httpWebResp = (HttpWebResponse)httpWebRequest.GetResponse();
                using (var streamReader = new StreamReader(httpWebResp.GetResponseStream()))
                {
                     strResult = streamReader.ReadToEnd();
                }

                return strResult;

            }
            catch (Exception ex)
            {
                string e = ex.Message;
            }

           return strResult;


        }
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