farm-2.x-dev/modules/taxonomy/log_category/farm_log_category.module

<?php

/**
 * @file
 * Contains farm_log_category.module.
 */

use Drupal\Core\Entity\EntityTypeInterface;

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

  // Add category base field to all log types.
  $fields = [];
  if ($entity_type->id() == 'log') {
    $category_info = [
      'type' => 'entity_reference',
      'label' => t('Log category'),
      'description' => t('Use this to organize your logs into categories for easier searching and filtering later.'),
      'target_type' => 'taxonomy_term',
      'target_bundle' => 'log_category',
      'multiple' => TRUE,
      'weight' => [
        'view' => 80,
      ],
      'form_display_options' => [
        'type' => 'options_select',
        'weight' => 10,
      ],
    ];
    $fields['category'] = \Drupal::service('farm_field.factory')->baseFieldDefinition($category_info);
  }
  return $fields;
}