class OEmbedWidget

class OEmbedWidget extends StringTextfieldWidget {
  
  /**
   * {@inheritdoc}
   */
  public function formElement(FieldItemListInterface $items, $delta, array $element, array &$form, FormStateInterface $form_state) {
    $element = parent::formElement($items, $delta, $element, $form, $form_state);
    /** @var \Drupal\media\Plugin\media\Source\OEmbedInterface $source */
    $source = $items->getEntity()
      ->getSource();
    $message = $this->t('You can link to media from the following services: @providers', [
      '@providers' => implode(', ', $source->getProviders()),
    ]);
    if (!empty($element['value']['#description'])) {
      $element['value']['#description'] = [
        '#theme' => 'item_list',
        '#items' => [
          $element['value']['#description'],
          $message,
        ],
      ];
    }
    else {
      $element['value']['#description'] = $message;
    }
    return $element;
  }
  
  /**
   * {@inheritdoc}
   */
  public static function isApplicable(FieldDefinitionInterface $field_definition) {
    $target_bundle = $field_definition->getTargetBundle();
    if (!parent::isApplicable($field_definition) || $field_definition->getTargetEntityTypeId() !== 'media' || !$target_bundle) {
      return FALSE;
    }
    return MediaType::load($target_bundle)->getSource() instanceof OEmbedInterface;
  }

}