How to retrieve app user's orders from frontend widget

Hi there,
I developed an eCommerce app that’s pure HTML + JavaScript. In my test site it runs inside an iframe that I injected via “Tracking & Analytics”.

I set up my app server as explained in this tutorial:

When my eCommerce app loads up, it needs to access recent orders from my user’s database (where my app is installed).

But now, how do I access, in JavaScript, orders for a specific site (via my backend)? I read the SDK document and this one (Get Started with APIs | Developer Support Center | Wix.com), but I still don’t get how will the frontend component have the refresh_token to access the data it needs?
Put in another way - how do I now feed data about my user’s orders to my iframe in JavaScript now?

Thanks :slight_smile: