To show a duties estimate to international customers in your checkout, you need to edit your product details to include the country or region of origin and the HS code.
Keep in mind that the duty rate is calculated based on a number of factors, so they may not apply for certain combinations of HS codes/shipping destinations.
Now that you’ve added the HS code to that product, it should be coming through the InventoryItem/InventoryLevel API as harmonized_system_code.
Shopify currently doesn’t calculate duty estimates automatically for checkouts/orders, but you could use the above to show an estimate of the cost of duties for an order at checkout and calculating with a private app (or a third-party app from the Shopify app store that does the same), and then updating the order with the duties object with the calculated amount etc if needed.
We are aware of the benefits of this functionality if it were automatically calculated by Shopify, so I can log your feedback internally!