WishList Member doesn’t process the actual payments (that happens on the payment provider side) so there isn’t a coupon functionality built in to WishList Member.
But there is an option you can look into for using Coupons with WishList Member.
Coupons with Payment Provider:
Your payment provider handles the payments and WishList Member handles all the control regarding the access to your site content.
So if your payment provider allows for the use of coupons, you can look into using those to create coupons.
Options like Stripe and WooCommerce are popular options for using coupons.
If you are unsure, you can contact your chosen payment provider to see what they have for coupon options. Most will have something available.