code: public SignatureRequest sendSignatureRequest(SignatureRequest req) throws HelloSignException {
          if (req.hasId()) {
              throw new HelloSignException("Sending an existing signature request is not supported");
          }
          return new SignatureRequest(httpClient.withAuth(auth).withPostFields(req.getPostFields())
                  .post(BASE_URI + SIGNATURE_REQUEST_SEND_URI).asJson());
      }
docs: Sends the provided signature request to HelloSign.
  
  @param req SignatureRequest
  @return SignatureRequest
  @throws HelloSignException thrown if there's a problem processing the
  HTTP request or the JSON response.