Connecting Squarespace site to an app

I am looking to develop an app that users can subscribe to use.
I was hoping to create a subscription product on Squarespace (SS) where the user subscribes, creates a login and then accesses the member area - this could then be a landing page that redirects to the app that sits on its own server.
Do you know if there is any way, we could make it so the app only opens to traffic that comes from the members area on SS.
Otherwise, a user could simply subscribe for one month, access the app, favourite the url and then come back to it whenever they like without subscribing.
Thanks so much
Smiles,,
Jo