addFormElements example

// The node is not part of any book yet - set default options.       $this->entity->book = $this->bookManager->getLinkDefaults($this->entity->id());
    }
    else {
      $this->entity->book['original_bid'] = $this->entity->book['bid'];
    }

    // Find the depth limit for the parent select.     if (!isset($this->entity->book['parent_depth_limit'])) {
      $this->entity->book['parent_depth_limit'] = $this->bookManager->getParentDepthLimit($this->entity->book);
    }
    $form = $this->bookManager->addFormElements($form, $form_state, $this->entity, $this->currentUser(), FALSE);

    return $form;
  }

  /** * {@inheritdoc} */
  protected function actions(array $form, FormStateInterface $form_state) {
    $actions = parent::actions($form, $form_state);
    $actions['submit']['#value'] = $this->entity->book['original_bid'] ? $this->t('Update book outline') : $this->t('Add to book outline');
    $actions['delete']['#title'] = $this->t('Remove from book outline');