Sync Main Site To Webapp Site

What I am wanting to do is to have our main website, https://feastthailand.com sell access to the webapp we have created, which is https://feastfinder.co. On checkout, users will create an account on our main website and I then want the user details to be passed to the webapp site so purchasers can immediately be able to login to the app.

In WP Remote Users Sync, what exactly do I need to set on the main website and what on the webapp site?

The page I need help with: [log in to see the link]