Since PayPlans 4.2.3, it supports passing of coupon codes via the URL parameter. You can share the checkout link to your customers by using the URL below:
http://yoursite.com/index.php?option=com_payplans&task=plan.subscribe&**plan_id=XX**&**coupon_code=YY**
Discounts API Parameters
plan_id : This would be the plan id that you would want the user to subscribe to
coupon_code : This would be the coupon code that you want to apply to the checkout automatically