As long as you add that coupon code to your subscription order before the code expires you will be fine!

Even if your next subscription date is passed the date of expiry for the coupon code, it will still work.

Here is a quick article on how to add a coupon code to a subscription order: https://support.annmariegianni.com/hc/en-us/articles/360035580533-How-to-Add-a-Coupon-Code