Note: The selected products must be out of stock to start showing Back In Stock function
Steps
From left-sidebar on the app admin page → Click Back in stock → Click Settings → Click Conditions
On Show back in stock with products section, tick on Choose products
Click on Search box, search or select the product(s) from the store product list you want to show up the BIS function
Click Save to save changes
Note: the selected products are still hidden if they belong to the collection/tag(s) you set up to hide the Back In Stock function. For your reference https://globo.io/kb/hide-the-back-in-stock-button-for-specific-out-of-stock-products