If you have existing users that you would like to offer a coupon, you can apply a coupon code to an existing user's subscription plan.
To do so, navigate to the People tab, and under the second People tab, find the user for whom you will apply the coupon and click Edit. Go to the Subscription tab to view their active subscription plan. Click the 3 vertical dots option and select Apply coupon. Choose from your list of coupons which one you will offer and select Apply.
You'll see the coupon applied on the selected subscription plan.