function wp_use_cf_client_ip($comment_author_ip) {
    if(isset($_SERVER['HTTP_CF_CONNECTING_IP'])) {
           return $_SERVER['HTTP_CF_CONNECTING_IP'];
    }

    // return defalt otherwise
    return $comment_author_ip;
}

add_filter('pre_comment_user_ip', 'wp_use_cf_client_ip');