Strategies to create a waiting list/ email flow for customers, say if an item is out of stock.

Hi team, 

I am in the midst of planning a waiting list email marketing strategy for our brands. Say if an item is out of stock, and when people click on the ‘Join the waiting list’ button, they will instantly receive an email when the item is back on stock. 

May I know what are the steps to implement such strategy on Klaviyo? I reckon a pop-up is a must, for people to join the waiting list. But the in stock/ out of stock status is a tricky one, how does Klaviyo recognizes if an out-of-stock item is back in stock, and then trigger it to send out a dynamic email to the particular person who joined the waiting list? 

Would love to learn more from the experts! As I am relatively new to email marketing. Thanks!