Add this code to the Theme’s functions.php file to show the wishlist ID and add some HTML in your single wishlist page.
add_action('wishlist_single', 'wishlist_single_20201908', 0);
function wishlist_single_20201908($atts){
$wishlist_id = isset( $atts['id'] ) ? $atts['id'] : 0;
echo '<pre>'.var_export($wishlist_id, true).'</pre>';
?>
<p>Custom HTML here</p>
<?php
}

