It's normally the responsibility of the app to 'register' with the OS for different capabilities that it supports/provides - it's not generally something that you can do manually, although there may be settings within the app that expose that. I don't use PocketCasts so I can't help you out there, but you could wing them an email and ask about adding that support - it's probably just a couple lines of code in the Manifest.
EDIT: I installed PocketCasts and Pocket and neither of them show up in the app list, which seems a bit strange since they do stream audio - an email to the developer, including a screenshot, makes the most sense to get that added.
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