Recaptcha can be enabled, get the keys here. Then add/change the following to your config.php file. The section is there by default, just without the sitekey and secret.

'captcha' => [
    'enabled' => true,

    'sitekey' => 'YOUR SITE KEY HERE',
    'secret' => 'YOUR SITE SECRET HERE',

You can also add additional attributes using the attribute key. See config/captcha.php to see details on what attributes you need to add to your config.php file.