This article applies to Discount Ninja version 3 and above
This feature is not available on the free plan
Discount Ninja can be configured to hide or show a discount field at checkout.
The settings related to this feature can be found under Settings > Integration > Sales channel.
Be careful when you enable this field though.
Discount codes entered at checkout compound with the discounts already applied by Discount Ninja.
Here's a demo of exactly how the discount field on the checkout works: