farm-2.x-dev/modules/core/log/modules/asset/farm_log_asset.module

<?php

/**
 * @file
 * Contains farm_log_asset.module.
 */

use Drupal\Core\Entity\EntityTypeInterface;

/**
 * Implements hook_entity_base_field_info().
 */
function farm_log_asset_entity_base_field_info(EntityTypeInterface $entity_type) {

  // We only care about log entities.
  if ($entity_type->id() != 'log') {
    return [];
  }

  // Add an asset reference field to logs.
  $field_info = [
    'type' => 'entity_reference',
    'label' => t('Assets'),
    'description' => t('What assets do this log pertain to?'),
    'target_type' => 'asset',
    'multiple' => TRUE,
    'weight' => [
      'form' => 50,
      'view' => -10,
    ],
  ];
  $fields['asset'] = \Drupal::service('farm_field.factory')->baseFieldDefinition($field_info);
  return $fields;
}