How can I set up a coupon campaign for new subscriptions only?

This article explains how to set up coupon campaigns for new subscriptions only. 

How to set up coupon campaigns only for new subscriptions on the dashboard

Go to the Offers & Coupons section of the platform, and then to the Create Coupons section of coupon management.

Here you can create your campaign as usual, with one small change. When setting the offers for Campaign applicable on, choose subscription offers (recurring billing) from the drop-down and the Only new subscriptions option. 

Now you can create a campaign that will allow your existing customers to apply a coupon.