Custom remote video thumbnail
I solved the problem by adding an image field on the remote video media entity and then altering the media entity on presave, like this:

/**
 * Implements hook_entity_presave().
 */
function MY_MODULE_entity_presave(Drupal\Core\Entity\EntityInterface $entity) {
  if ($entity-&gt;getEntityTypeId() == 'media' &amp;&amp; isset($entity-&gt;custom_thumbnail-&gt;entity)) {
    $entity-&gt;thumbnail-&gt;target_id = $entity-&gt;field_media_image-&gt;target_id;
  }
}


The image field is called field_media_image.