The Bookly Waiting List (Add-on) allows you to enable this feature in your Bookly booking system.
Demo: https://codecanyon.net/item/bookly-waiting-list-addon/20917406
Let customers subscribe to a mailing list for out-of-stock or unavailable products so they can get automatic back-in-stock notifications when these are available in your shop.
Demo: https://yithemes.com/themes/plugins/yith-woocommerce-waiting-list/
YITH WooCommerce Waiting List Premium allows users to request an email notification when an item is out of stock in stock.
Demo: https://yithemes.com/themes/plugins/yith-woocommerce-waiting-list/