ContainerForm::form

public ContainerForm::form(array $form, FormStateInterface $form_state)

public function form(array $form, FormStateInterface $form_state) {
  $taxonomy_term = $this->entity;
  // Build the bulk of the form from the parent forum form.
  $form = parent::form($form, $form_state, $taxonomy_term);

  // Set the title and description of the name field.
  $form['name']['#title'] = $this->t('Container name');
  $form['name']['#description'] = $this->t('Short but meaningful name for this collection of related forums.');

  // Alternate description for the container parent.
  $form['parent'][0]['#description'] = $this->t('Containers are usually placed at the top (root) level, but may also be placed inside another container or forum.');
  $this->forumFormType = $this->t('forum container');
  return $form;
}