EntityTestExternal.php

<?php

namespace Drupal\entity_test\Entity;

use Drupal\Core\Url;

/**
 * Test entity class.
 *
 * @ContentEntityType(
 *   id = "entity_test_external",
 *   label = @Translation("Entity test external"),
 *   base_table = "entity_test_external",
 *   entity_keys = {
 *     "id" = "id",
 *     "uuid" = "uuid",
 *     "bundle" = "type",
 *   },
 *   links = {
 *     "canonical" = "/entity_test_external/{entity_test_external}"
 *   },
 * )
 */
class EntityTestExternal extends EntityTest {
    
    /**
     * {@inheritdoc}
     */
    public function toUrl($rel = NULL, array $options = []) {
        if ($rel === 'canonical') {
            return Url::fromUri('http://example.com', $options);
        }
        return parent::toUrl($rel, $options);
    }

}