Whitelist FB UA

<?php
/**
 * Plugin Name: MH Captcha - Facebook UA whitelist
 * Description: Trust requests from Facebook link preview scrapers (Facebot / facebookexternalhit).
 */

add_filter('mhcaptcha/is_trustable_request', function ($is_trustable) {
    $ua = $_SERVER['HTTP_USER_AGENT'] ?? '';

    // Facebook scrapers typically use these user agents.
    if ($ua && (
        stripos($ua, 'facebookexternalhit') !== false ||
        stripos($ua, 'Facebot') !== false
    )) {
        return true;
    }

    return $is_trustable;
});