In the embed code you can add a dynamic client-reference-id parameter or prefill email with your logged in user. If you require a user to be logged in before subscribing - just overlay the whole pricing table with a free positioned (transparent) frame and make it conditional based on the user being logged in.
We use this method for our own pricing and subscription.
Custom Cards
For more advanced use cases, where you have numerous subscriptions to choose from or want to conditionally display certain prices and plans - configure the a subscription interval to indicate the product is a subscription. This works the same as a checkout, except you will add an Interval field in the