There are two APIs available.
One is for creating, updating and deleting instances. With this API you can also manage your team.
Then we have a second API which is per instance, so you use this API for managing users and ACL rules per instance.
Both uses an api key sent as password in Basic Auth, but please keep in mind to use correct key for each of the api since they don't share key.
Documentation can be found here: https://docs.cloudmqtt.com