Hi @Mike McNulty ,
Thank you for posting query in Microsoft Q&A Platform.
As described in documentation Pre-requisites section, we need to AAD application ID(Service principle) with secret and then add that application id to security group of your intended as member. Now, this security group only we will be adding to Kafka REST Proxy as described here.
Now, once we done with above in the client application code, we will be using application ID(service principle) and secret what we created above to interact with cluster and APIs.
So, that way our client application will able to interact with APIs as client application will use service principle to do same. and service principle was part of security group which added under Kafka REST Proxy enable settings.
Hope this helps. Please let us know how it goes. Thank you.