code: public FileUrlResponse getFilesUrl(String requestId) throws HelloSignException {
          String url = BASE_URI + SIGNATURE_REQUEST_FILES_URI + "/" + requestId;
  
          HttpClient httpClient = this.httpClient.withAuth(auth).withGetParam(PARAM_GET_URL, "1").get(url);
  
          if (httpClient.getLastResponseCode() == 404) {
              throw new HelloSignException(String.format("Could not find request with id=%s", requestId));
          }
  
          return new FileUrlResponse(httpClient.asJson());
      }
docs: Retrieves a URL for a file associated with a signature request.
  
  @param requestId String signature request ID
  @return {@link FileUrlResponse}
  @throws HelloSignException thrown if there's a problem processing the
  HTTP request or the JSON response.
  @see <a href="https://app.hellosign.com/api/reference#get_files">https://app.hellosign.com/api/reference#get_files</a>