The app works based on a script tag and should be loaded and running after clicking Enable in the top section of the app page:


To test the app, create a pre-order offer, and make sure you enabled preorder on your products, as noted here:

Once your offer is activated you should see the preorder button in your product page.

If you're not seeing it, you might need to add a custom selector to help the app identify your add to cart button: