Discount codes for single payments are set in the Discounts section inside DepositFix. When creating a new discount, you can set the discount code and the discount amount. The latter can be a fixed amount, a percentage discount, or a number of trial days (that means offsetting the charge for a certain amount of days). It's also possible to set an expiration date, the number of redemptions or tie the discount to just one specific form.
The discounts you set inside the DepositFix app don't apply to subscriptions, but only to single payment products. Discounts for subscriptions can only be set in Stripe. The setup is very similar to single payment products, only that "Discounts" are called "Coupons" in Stripe. The rest is pretty much the same - all you have to do is set the discount code, the discount type and the amount.