First, to enable Apsis as a Subscription Plugin you will need some information from Apsis. This information is a little bit hard to find, that's why this article exists. This is the information you'll need:
These fields contains the apsis login and the MailList-IDs for order placement and regular sign up (by a form on your startpage etc).
Finding the MailingListID
First, press "Help & Support".
Then press the API-link below Guides, it'll expand.
Select the item regarding "fetching mailing lists".
This one explains how you connect the systems.
Press the first link inside the article:
This will open up a form where you're able to insert your credentials. Do it and then press Invoke.
You will receive a bunch of data, look for the thing called MailingListID that refers to the list you want, in black you'll see the correct MailingListID.
Insert that ID into the List ID-field inside the Centra Plugin Setup. You are now connected with Apsis!