add_action('woocommerce_checkout_update_order_meta','save_additional_info_to_order_meta');

function save_additional_info_to_order_meta($order_id) {
    $array_options = ['search','affiliate_link','video','ad'];
    if ( ! empty($_POST['referral_info']) && in_array ($_POST['referral_info'], $array_options)) {
        $order = wc_get_order($order_id);
        $order->update_meta_data('Referral', sanitize_text_field($_POST['referral_info']));
        $order->save_meta_data();
    }
}