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);
  }

}