...
Stores > Configuration > Customers > Wishlist
Under the additional Additional Data tab, Is Enabled should be set to yes
...
Ajax add-to-wishlist works by default after the module is are installed. (see view/frontend/web/js/wishlist/add-to-wishlist-ext.js file)
...
After clicking on the wishlist icon selected class is added to clicked cs-links-block-addto__link The same class is added to every wishlist link that is already in the given user wishlist. (see view/frontend/web/js/wishlist/is-in-wishlist.js file)
...