In the latest version, we have added support to save custom URLs on the wishlist, by default all post types, categories, tags, custom taxonomies, the author will save by an ID, and any other page WordPress itself like search page, date, month, year, 404, etc will convert an ID to save wishlist. we have used a custom database table (wp_pickplugins_wl_urlid) to save custom URL to convert and id. so basically you can save any other URL / external URL to save on wishlist by saving on a custom database table.
you can use as following to save custom URL to wishlist
[wishlist_button id="https://pickplugins.com/url-to-wishlist" obj_type="url" show_count="yes" show_menu="yes" icon_active="" icon_inactive="" icon_loading=""]