add_action('wishlist_single_main', 'wishlist_single_main_20201908', 0);
function wishlist_single_main_20201908($atts){
$wishlist_id = isset( $atts['id'] ) ? $atts['id'] : 0;
//echo '<pre>'.var_export($wishlist_id, true).'</pre>';
?>
<p>Custom HTML here</p>
<?php
}