sync.php

use Drupal\Core\Routing\RouteMatchInterface;
use Drupal\Core\Entity\EntityInterface;
use Drupal\Component\Utility\Html;
use \Drupal\node\Entity\Node;


function relater_node_presave(EntityInterface $node) {
    if ($node->bundle() == 'my_custom_node') {
        $embeds = _relater_get_embeds($node);
        $node->field_my_entityref->setValue($embeds);
    }
}

/**
 * @param EntityInterface $node
 * @return array
 */
function _relater_get_embeds(EntityInterface $node) {
    $entities = array();
    $text = $node->field_description[0]->value;

    // Our parsing is cribbed from EntityEmbedFilter's process function.
    if (strpos($text, 'data-entity-type') !== FALSE && (strpos($text, 'data-entity-embed-display') !== FALSE || strpos($text, 'data-view-mode') !== FALSE)) {
        $dom = Html::load($text);
        $xpath = new \DOMXPath($dom);

        foreach ($xpath->query('//drupal-entity[@data-entity-type and (@data-entity-uuid or @data-entity-id) and (@data-entity-embed-display or @data-view-mode)]') as $embed) {
            /** @var \DOMElement $embed */
            $entity = NULL;
            try {
                $id = $embed->getAttribute('data-entity-id');
                $entity = Node::load($id);
                if ($entity) {
                    $entities[] = array('target_id' => $entity->id());
                }
            } finally {
                // We're going to rudely ignore this case.
            }
        }
    }

    return $entities;
}