Google Tag Manager API v2

In Google Cloud Console, create or select a project, enable the Tag Manager API if needed, then create OAuth 2.0 credentials under APIs & Services → Credentials. Use Google's OAuth flow to obtain an access token with Tag Manager scopes such as https://www.googleapis.com/auth/tagmanager.readonly or the narrower write scopes documented in the API discovery/auth pages.

In Google Cloud Console, create or select a project, then create an API key under APIs & Services → Credentials. The Tag Manager discovery document says the key query parameter identifies your project and is required unless you provide an OAuth 2.0 token.