code: @PostMapping("/subscribe")
      public final void subscribeAppToPage() {
          MultiValueMap<String, String> params = new LinkedMultiValueMap<>();
          params.set("access_token", getPageAccessToken());
          restTemplate.postForEntity(fbApiEndpoints.getSubscribeUrl(), params, String.class);
      }
docs: Invoke this method to make the bot subscribe to a page after which
  your users can interact with your page or in other words, the bot.
  <p>
  NOTE: It seems Fb now allows the bot to subscribe to a page via the
  ui. See https://developers.facebook.com/docs/messenger-platform/getting-started/app-setup