Checkout UI extension line item

Retired Boards Appdev Custom Storefronts
1

I’m trying the Checkout UI extension which is working good on my demo store for basic components.

I can’t figure how to add a line item like on the demo here: https://shopify.dev/api/checkout-extensions/checkout

I can’t find any sample code or anything that can help - I’m working with Vanilla JS.

Does someone have any clue?

Thanks.