Retrieving the user ID in authentication using proxy server method

Context:

I am building an application which communicates with the backend server. To authenticate the request, I am planning use the secure setting method. Now, while installing the App, we provide the API key. Then client will make a request to the backend using the proxy server. And the proxy server attaches the API Key to the request.

The backend needs an API Key to validate the request and returns with an auth token. This auth token will have the user permissions, so the backend also needs to know the User ID.

Question:

Since my app is installed for every user of the instance, I need to generate the auth token which has restrictions depending on the user-id. So, will the zendesk proxy server attach some userId to the request? If so, is it guaranteed to not change in future? PS: I saw x-zendesk-user-id in the request but didn't find this in any documentation.