When you create your coupon codes, the default setting will be to apply the discount evenly across all installments, whether you have a payment plan or not on the invoice.
You can choose to prevent the code being applied on the first installment, which will then apply the discount only to the subsequent installments.

For example, this is a good option if you have an invoice with two payments, a deposit and the remaining balance. In this example, if you have set up the coupon code to prevent the discount on the first installment, the discount will not be applied to your deposit installment, only to the remaining balance installment.

Note: Coupon codes only work with Square as the payment processor. All other processors will be disabled.