code: public HttpClient post(String url) throws HelloSignException {
          if (getParams != null) {
              logger.warn("GET parameters set for a POST request, they will be ignored");
          }
          request = new HttpPostRequest(url, postFields, auth);
          request.execute();
          return this;
      }
docs: Initializes a POST request to the given URL.
  
  @param url String url
  @return HttpClient
  @throws HelloSignException thrown if the url is invalid