code: public A callbackUri(String callbackUri) {
          this.headers.add(Pair.of("X-CallbackURI", callbackUri));
          return (A)this;
      }
docs: Sets the notification channel URI that the registered callback message
  will be sent to.
  
  When using an authenticated web service, this parameter is required.
  
  @param callbackUri   the notification channel URI
  @return  this