From Shopify admin

2. Click product you want to manage

3. Already have a subscription plan and you want to add it to this product? Great, you can select it from a list.

In the Subscriptions section, click Add Option and Select Existing Option

Select subscription plan from drop-down menu and click Add to Plan to save changes. This will save

If you don't have a subscription plan yet created for this product, you will need to add one.

In the Subscriptions section, click Add Option and Add New Option

Fill out the information for your new subscription plan and click Save.