YouTube API for brand account

We are trying to build a internal tool using YouTube API to manage our YouTube channel. We are using G Suite (or Google Workspace), and our YouTube account is a brand account and in YouTube Partner Program.

But I cannot access our YouTube channel via YouTube API. I think there are two options to do that:

1. Authorize the application with YouTube Account. I can test it on Google OAuth 2.0 Playground and it returns correct results. But on our own application (Internal Mode on GCP), it cannot recognize the brand account as an internal account and return Error 403: org_internal or Error 403: access_denied. Is there a way to add the brand account as a test user? (it requires an email, but we cannot find it)

2. Authorize the application with a G suite account, and use onBehalfOfContentOwner in API calls. It requires a content owner id, but we cannot find it. We tried contentOwners API with fetchMine=true but got nothing. (A similar question: Where do I find my or my customers YouTube "content owner id"?)

Is there any other option to let the API access the YouTube channel of the brand account?