private String postRequest(String serviceSessionUrl, String jsonString) throws IOException {
    HttpEntity entity = MultipartEntityBuilder.create()
            .addTextBody("json", jsonString, ContentType.create("application/json", Charsets.UTF_8))
            .build();
    HttpPost post = new HttpPost(serviceSessionUrl);
    post.setEntity(entity);
    String result = null;
    try (CloseableHttpResponse response = httpClient.execute(post)) {
      result = EntityUtils.toString(response.getEntity());
    } catch (IOException e) {
      throwClientSideException(jsonString, serviceSessionUrl, e);
    }
    throwServerSideException(result, jsonString, serviceSessionUrl);
    return result;
  }