farm-2.x-dev/modules/core/ui/user/farm_ui_user.install

<?php

/**
 * @file
 * Install, update and uninstall functions for the farm_ui_user module.
 */

/**
 * Implements hook_install().
 */
function farm_ui_user_install() {

  // Set the default 403 page to /403.
  \Drupal::configFactory()->getEditable('system.site')->set('page.403', '/403')->save();
}