Hello shiafern-yu,
Currently, assigning a GST code for the Discounts is not possible. The calculation depends whether you want to Exclusive of Tax, Inclusive of Tax, or Out of scope of GST.
Regarding whether the discount is taxable or not, I'd recommend reaching out your accountant. They might be able to provide a different way to get this working according to your business setup. They know what's best for you and your books.
Please feel free to drop by your queries so we can review it for you. Have a great day!
