ext_localconf.php

<?php
defined('TYPO3_MODE') || die('Access denied.');

call_user_func(function () {
    \TYPO3\CMS\Core\Utility\ExtensionManagementUtility::addService(
        'mfc_belogin_captcha',
        'auth',
        \Mfc\MfcBeloginCaptcha\Service\CaptchaService::class,
        [
            'title' => 'Backend Login Captcha Service',
            'description' => 'Extends the authentication with a captcha protection after an amount of login failed',
            'subtype' => 'authUserBE',
            'available' => true,
            'priority' => 70,
            'quality' => 75,
            'os' => '',
            'exec' => '',
            'className' => \Mfc\MfcBeloginCaptcha\Service\CaptchaService::class,
        ]
    );

    /**
     * Here, the default login provider will be overridden
     * as we don't want to add a new kind of login but just
     * add a captcha to the username and password login.
     */
    $GLOBALS['TYPO3_CONF_VARS']['EXTCONF']['backend']['loginProviders'][1433416747]['provider'] =
        Mfc\MfcBeloginCaptcha\LoginProvider\MfcBeloginCaptchaLoginProvider::class;
});