This code is for creating a new attribute for the shortcode of a single wishlist.
add_filter('wishlist_single_atts', 'wishlist_single_atts_20201908', 5);
function wishlist_single_atts_20201908($atts){
$atts['custom_atts'] = 'Custom atts';
return $atts;
}
For Example:
[wishlist_button id="1234"]