From Shopify Admin:
From Shopify admin, click on Products
2. Click product you wish to manage
3. In the Subscriptions section, if you already have a subscription group applied to this product, you can click Edit.
Here you can add a subscription discount to this existing plan.
To create a new subscription plan to this product, click Add Option and then Add new option. Add your discount to this new plan.
To choose from an existing subscription plan you've created but haven't applied to this product, click Add Option and then Select Existing Option.
Select subscription plan and click Add to Plan to save changes.
Then click Edit on plan to add discount to subscription plan.