Search by order item SKU or ID in WooCommerce Orders Admin page

Question

What I am trying to do is to be able to search by order item SKU or ID in the WooCommerce Orders Admin page.

What I have found/done till now, but with no success is

Answer 1

@blacksquare, @jibby, @helgatheviking you are the men! This is the code that works, due to your help.

    //Search by product SKU in Admin Woocommerce Orders
add_filter( 'woocommerce_shop_order_search_fields', function ($search_fields ) {
    $posts = get_posts(array('post_type' => 'shop_order'));

    foreach ($posts as $post) {
        $order_id = $post->ID;
        $order = new WC_Order($order_id);
        $items = $order->get_items();

        foreach($items as $item) {
            $product_id = $item['product_id'];
            $search_sku = get_post_meta($product_id, "_sku", true);
            add_post_meta($order_id, "_product_sku", $search_sku);
        }
    }

    return array_merge($search_fields, array('_product_sku'));
});