A promotion code is entered in the cart, before checkout: the cart page has a field for it, and applying a valid code recalculates the totals immediately - you see the discount before paying anything.
Codes have terms set by the operator: some work only on the first invoice, some on every renewal, some only for specific products or billing cycles. If a code is refused, the cart says so rather than silently ignoring it.
One code applies per cart.