This feature is supported for merchants who have Shopify POS Android v3.1.3 or iOS v5.2.1 installed. You can learn more about it here.
We encourage you to try setting cart discounts with a code. We’d love to hear your feedback and how you’ll use this in your app. If you want to make a suggestion about this feature, or if you have any questions about it, then don't hesitate to contact the team at firstname.lastname@example.org.