class EntityDisplayModeAddForm

class EntityDisplayModeAddForm extends EntityDisplayModeFormBase {
    
    /**
     * The entity type for which the display mode is being created.
     *
     * @var string
     */
    protected $targetEntityTypeId;
    
    /**
     * {@inheritdoc}
     */
    public function buildForm(array $form, FormStateInterface $form_state, $entity_type_id = NULL) {
        $this->targetEntityTypeId = $entity_type_id;
        $form = parent::buildForm($form, $form_state);
        // Change replace_pattern to avoid undesired dots.
        $form['id']['#machine_name']['replace_pattern'] = '[^a-z0-9_]+';
        $definition = $this->entityTypeManager
            ->getDefinition($this->targetEntityTypeId);
        $form['#title'] = $this->t('Add new @entity-type %label', [
            '@entity-type' => $definition->getLabel(),
            '%label' => $this->entityType
                ->getSingularLabel(),
        ]);
        return $form;
    }
    
    /**
     * {@inheritdoc}
     */
    public function validateForm(array &$form, FormStateInterface $form_state) {
        parent::validateForm($form, $form_state);
        $form_state->setValueForElement($form['id'], $this->targetEntityTypeId . '.' . $form_state->getValue('id'));
    }
    
    /**
     * {@inheritdoc}
     */
    protected function prepareEntity() {
        $definition = $this->entityTypeManager
            ->getDefinition($this->targetEntityTypeId);
        if (!$definition->get('field_ui_base_route') || !$definition->hasViewBuilderClass()) {
            throw new NotFoundHttpException();
        }
        $this->entity
            ->setTargetType($this->targetEntityTypeId);
    }

}