public void GetParticipantJsonFile(string surveyKey)
{
    var attribute = new string[] { "attribute_1", "attribute_2" };
 
    this.Client.ClearParameters();
    this.Client.Method = "list_participants";
    this.Client.Parameters.Add("ssessionkey", this.SessionKey);
    this.Client.Parameters.Add("isurveyid", surveyKey);
    this.Client.Parameters.Add("aAttributes", JArray.FromObject(attribute));
    this.Client.Parameters.Add("bUnused", false);
    this.Client.Post();
 
    //save in file
    File.WriteAllBytes(@"C:\Users\xxxx\Desktop\test.json", this.Client.Response.GetFileBlob());
}

public JsonRPCResponse Post()
{
    JObject jobject = new JObject();
    jobject.Add(new JProperty("jsonrpc", "2.0"));
    jobject.Add(new JProperty("id", ++id));
    jobject.Add(new JProperty("method", Method));
    jobject.Add(new JProperty("params", Parameters));
 
    string PostData = JsonConvert.SerializeObject(jobject);
    UTF8Encoding encoding = new UTF8Encoding();
    byte[] bytes = encoding.GetBytes(PostData);
 
    HttpWebRequest request = (HttpWebRequest)WebRequest.Create(URL);
    request.Method = "POST";
    request.ContentType = "application/json";
    request.KeepAlive = true;
    request.ContentLength = bytes.Length;
 
    Stream writeStream = request.GetRequestStream();
    writeStream.Write(bytes, 0, bytes.Length);
    writeStream.Close();
 
    HttpWebResponse response = (HttpWebResponse)request.GetResponse();
    Stream responseStream = response.GetResponseStream();
    StreamReader readStream = new StreamReader(responseStream, Encoding.UTF8);
 
    Response = new JsonRPCResponse();
    Response = JsonConvert.DeserializeObject<JsonRPCResponse>(readStream.ReadToEnd());
    Response.StatusCode = response.StatusCode;
 
    return Response;
}

{
    "jsonrpc": "2.0",
    "id": 2,
    "method": "list_participants",
    "params": {
        "ssessionkey": "xxx",
        "isurveyid": "xxx",
        "aAttributes": [
            "attribute_1",
            "attribute_2"
        ],
        "bUnused": false
    }
}

{
    "tid": "xxx",
    "token": "xxx",
    "participant_info": {
        "firstname": "xxxx",
        "lastname": "xxx",
        "email": "xxx@xxx.x"
    }
}

    this.Client.Parameters.Add("ssessionkey", this.SessionKey);
    this.Client.Parameters.Add("isurveyid", surveyKey);
    this.Client.Parameters.Add("istart", 0);
    this.Client.Parameters.Add("ilimit", 10);
    this.Client.Parameters.Add("bUnused", false);
    this.Client.Parameters.Add("aAttributes", JArray.FromObject(attribute));