farm-2.x-dev/modules/core/owner/farm_owner.module

<?php

/**
 * @file
 * Contains farm_owner.module.
 */

use Drupal\Core\Entity\EntityTypeInterface;

/**
 * Implements hook_entity_base_field_info().
 */
function farm_owner_entity_base_field_info(EntityTypeInterface $entity_type) {
  $fields = [];

  // Add owner field to logs and assets.
  if (in_array($entity_type->id(), ['asset', 'log'])) {
    $field_info = [
      'type' => 'entity_reference',
      'label' => t('Owners'),
      'description' => t('Assign ownership to one or more users.'),
      'target_type' => 'user',
      'multiple' => TRUE,
      'weight' => [
        'form' => -70,
        'view' => -70,
      ],
    ];

    $fields['owner'] = \Drupal::service('farm_field.factory')->baseFieldDefinition($field_info);
  }

  return $fields;
}