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;
    }

}