code: protected final ResponseEntity<Response> setGetStartedButton(String payload) {
          Event event = new Event().setGetStarted(new Postback().setPayload(payload));
          return restTemplate.postForEntity(fbMessengerProfileUrl, event, Response.class);
      }
docs: Call this method with a {@code payload} to set the "Get Started" button. A user sees this button
  when it first starts a conversation with the bot.
  <p>
  See https://developers.facebook.com/docs/messenger-platform/discovery/welcome-screen for more.
  
  @param payload for "Get Started" button
  @return response from facebook