Adding A Custom Product To Cart From An External Web App

[AnalyzingAnt111]

Hello,

I'm trying to build the following:

1. Button in our Shopify site that redirects user to an external web app.
2. In that app the user fills out specific product dimensions and views a preview design.

3. After confirming this product, they're then redirected back to Shopify with the new 'one-time' product added to their cart.

I'm stuck at the point of attempting to pass a cartId or user information necessary to create a new cart to the external app. The reason the app is external is that it's somewhat complicated design software and the price may vary drastically depending on the users design choices.

Has anyone encountered something like this before, or perhaps another way of achieving the same result?

Thanks a lot.