Preview woocommerce thank you page for an order in the Woocommerce order view

functions.php

<?php

/**
* Add a button to the order admin panel below the order details that opens the thank you page for that order.
*/
function action_woocommerce_admin_order_data_after_order_details( $order ) {
    echo '<a href="' . $order->get_checkout_order_received_url() . '" target="_blank" class="button" style="margin-top:10px">' . __( 'Preview Thank you page', 'woocommerce' ) . '</a>';
}
add_action( 'woocommerce_admin_order_data_after_order_details', 'action_woocommerce_admin_order_data_after_order_details', 10, 1 );