If you're using a theme that supports Themes 2.0 - follow this setup guide:

(1) Go to Online Store > Themes > Customize

(2) Head over to the Theme Settings > App Embeds:

(3) Activate the app:

(4) Save your changes.

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.