I see this in https://wiki.foxycart.com/v/2.0/products/subscriptions
Discounted subscription cost for only a certain number of renewals
You may need to be able to offer customers a special offer of receiving their subscription at a discounted price for the first X renewals, before the subscription becoming full price again.
To achieve that, you would make use of our coupons functionality and the setting to limit the number of uses per customer for the coupon. This restriction is based on the customers email address, so if you need to allow the customer to receive the first 3 payments on their subscription at a discounted rate, you would set the limit per customer to be 3.
For the customer to receive the discount, they would need to enter the coupon code on the cart or checkout. If you wanted them to receive the discount automatically, you could include the coupon parameter as part of the add to cart for your subscription, like coupon=my_coupon_code
But I don't see anything about using discounts on subscriptions. Is it possible?