How can I implement a coupon code on a monthly subscription product?

how can I implement a coupon code on a monthly subscription product? I know how to do it on a one time payment but i have a monthly recurring fee that I wanted to do a promo on and needed the customer to enter a coupon code. 

You can assign coupon code to specific product only, please check my screenshot below for reference:

Let us know if you are referring to something else.