class ContainerForm

class ContainerForm extends ForumForm {
    
    /**
     * Reusable URL stub to use in watchdog messages.
     *
     * @var string
     */
    protected $urlStub = 'container';
    
    /**
     * {@inheritdoc}
     */
    public function form(array $form, FormStateInterface $form_state) {
        // Build the bulk of the form from the parent forum form.
        $form = parent::form($form, $form_state);
        // 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;
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildEntity(array $form, FormStateInterface $form_state) {
        $entity = parent::buildEntity($form, $form_state);
        $entity->forum_container = TRUE;
        return $entity;
    }

}