How to create a discount code for active subscribers?

To create a discount code only for active subscribers, first turn on the feature to tag active subscribers with a specific tag in Seal Subscriptions > Settings > General Settings > Customers.
Then, go into Shopify Admin > Customers and filter customers by the tag you set the app to tag active subscribers with (e.g. seal_active_subscriber).
Name the filter as Subscribers and save the filter by clicking the Save filters button.

Go into Shopify Admin > Discounts > click Create discount and select the discount code type.
Scroll down to the Customer eligibility card and select Specific groups of customers option.
Select the Subscribers group (it can take a few minutes to see the newly created group here).
Save the discount.
Voila, that's it!