Discount code should have options like on shopify dashboard. On the dashboard you have the option for subscription purchases to
  1. Limit number of times this discount can be used in total
Limit discount to the first payment
Limit discount to multiple recurring payments
Discount applies to all recurring payments
This cannot be done with a unique code but if you guys can generate one that has the option to apply the discount in specific scenarios like above that would be very useful. Thank you.