Unfortunately, custom deals will not currently work with subscription-based orders. Our app uses draft orders for custom deals, which aren't supported in Subscription API. We are looking for a solution around this in a future release.
Is it possible to apply the discount code while bypassing the cart?
Skipping the cart is okay since ADG triggers on any webpage. If customers meet requirements on a product page, the discount works. Adding to the cart before checkout is best. But skipping the cart and going to checkout directly avoids the trigger, risking no discount. This ensures only eligible customers get the discount.