WooCommerce Subscriptions: How to auto-apply coupon for renewal orders only

Question: re: WC Subscriptions.

I want to be able to offer an automatic discount for WC Subscriptions renewal orders, but not have the discount applied to the very first original order.

I’m thinking the best way would be to create a coupon, and then auto-apply that coupon to the order before checkout if the order contains a renewal for the subscription. But I don’t know what the hook would be for that. Any ideas how I could go about this and what the right hooks would be?