Redirect to External URL in External Products - Woocommerce

functions.php

<?php 

function redirectExternalProduct()
{
    if (is_single() AND !empty(get_post_meta(get_the_ID(), '_product_url'))) {
        wp_redirect(get_post_meta(get_the_ID(), '_product_url', true));
        exit;
    }
}

add_action('wp', 'redirectExternalProduct');