How can I track subscriptions / purchases which are made over promotional offers on the appstore / product page?

We're currently promoting two subscriptions on the appstore product page as suggested here: https://developer.apple.com/app-store/promoting-in-app-purchases/

Both subscription products are also available within the app. Now I'd like to distinguish between purchases made within the app and triggered by the offers "outside" of the app on the product page. Is this possible? How?