Set up to start pre-order only when the quantity reaches zero

Steps

1. From the app admin page → Click Pre-order
2. Click Single Product or Multiple Products tab
3. Select a particular profile
4. In section CONDITION TO START/FINISH PREORDER → Select Pre-Order only when Out of Stock checkbox
5. Click Save to finish

Note: You must enable “Continue selling when out of stock” to allow pre-ordering products when sold out