When Pre-Order is turned on for an item, the app will replace the Add to Cart button to "Pre-Order" whenever available stocks in the inventory runs out (0 available stocks).
In order for the Pre-Order button to show, there must be no available stock in your inventory for this item:
Whenever the Pre-Order is enabled via the Pre-Order Manager app, the Continue selling when out of stock will be automatically enabled in the Products page. Should you disable the Pre-Order via the app, this option will be automatically disable itself.
