ext_tables.php

<?php
defined('TYPO3_MODE') or die();

// Add css file for YubiKey backend login
$TBE_STYLES['stylesheet2'] = '../typo3conf/ext/sf_yubikey/Resources/Public/Css/sf_yubikey.css';

// Add YubiKey fields to user settings
$GLOBALS['TYPO3_USER_SETTINGS']['columns']['tx_sfyubikey_yubikey_enable'] = [
    'label' => 'LLL:EXT:sf_yubikey/Resources/Private/Language/locallang_db.xlf:users.tx_sfyubikey_yubikey_enable',
    'type' => 'check',
    'table' => 'be_users'
];

$GLOBALS['TYPO3_USER_SETTINGS']['columns']['tx_sfyubikey_yubikey_id'] = [
    'label' => 'LLL:EXT:sf_yubikey/Resources/Private/Language/locallang_db.xlf:users.tx_sfyubikey_yubikey_id',
    'type' => 'user',
    'userFunc' => \DERHANSEN\SfYubikey\Hooks\UserSettings::class . '->user_yubikeyId',
    'table' => 'be_users'
];

\TYPO3\CMS\Core\Utility\ExtensionManagementUtility::addFieldsToUserSettings(
    '--div--;YubiKey,tx_sfyubikey_yubikey_enable, tx_sfyubikey_yubikey_id'
);