Zenamu will bill them at the original (non-discounted) rate for their second month and all subsequent renewals.
Discount codes are one-time only and apply only to the initial purchase. All renewals are billed at full price.
Can a client use a promo code when renewing an expired membership?
If a membership has expired and the client wants to purchase it again, they can use a new promo code during the new purchase — since it's treated as a new order, not a renewal.
The promo code field is available during the checkout process when buying a new membership. It is not shown during automatic recurring renewals, as those are processed at the full price.