Making a companion App

Hey folks,
so I want to do the following. I developed a Cloud app and now I want to release a “Companion App” for Android and ios, which can communicate with my API. There are several problems which arise from that.

First of all the Login of the User itself. I used oAuth this guide to authenticate the user, which works quite flawlessly.

But now the real problem arises. I don’t have the clientKey for the instance, which the user selected, so I can not identify the request. If I have done my research correctly I need a JWT which needs the clientKey. Is there any way I can get the clientKey for a given cloudId? Or is there any other way my companion app can act on behalf of the cloud instance, in which the user logged in during the oAuth process? Or is there simply no way for now, to use something like a companion app for a cloud addon?

All help is really appreciated!