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Currently, there is no add-on product page in the Microsoft Store. So it is expected that you can't find such a page. As you've already known, you could only choose the add-on product to be displayed in the parent product’s Microsoft Store listing or hide the add-on product.
To make the add-on product discoverable, you need list your add-on products in your app and using StoreContext API for purchasing.
Thank you.
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