If a user knows the coupon code, then before clicking on a payment button he can enter it into the special field located under the list of subscriptions:
For example, the coupon's settings are as follows:
The coupon is not bound to any plan or a user, so it can be used for any plan and by any user.
Suppose, the user intends to buy a plan that costs $100. The coupon gives him a 10% discount. Therefore, if the user enters the coupon code when purchasing the subscription, the subscription will cost him $90:
User's subscription history: