I am using the new Shopify customer accounts and can't seem to be able to add a link to the customer portal there

The new Shopify customer accounts aren't yet compatible with any apps. You can see this listed here: https://help.shopify.com/en/manual/customers/customer-accounts
This is why we can't add a link to the customer portal to that account type. 

A possible solution for this is to either:

a.) Switch back to the classic Shopify customer accounts to which our app can add a link to the customer portal

OR

b.) Use the subscription login link (accessible on /a/subscriptions/login page) which allows your customers to request a magic login link which allows them to edit their subscriptions.

OR

c.) You can also add a link to /a/subscriptions somewhere in your shop menu, so that when the customer logs in, they can see their subscription through this link.