Does Back in Stock work with the Shopify Buy Button?
The Shopify Buy Button allows you to add a 'Buy' button on pages that aren't hosted on Shopify.
If you are using the Buy Button on a non-store page (for example, a Wordpress site) then the Back in Stock widget won't work on that page.
If you are using the Buy Button on a static page on your Shopify store you can use the Back in Stock widget by adding your own link to the page. You can use the the same procedure described for collection pages: Add a Back in Stock button to the Collection page