On the Premium Plan, you could authenticate using one of two mechanisms:
API key or Client ID + Digital Signature
To manage your Client ID, visit Support Portal. However, after a Premium Plan project has migrated to Maps Platform, many of the menu options disappear including the link to Maps: Manage Client ID. To manage authorized URLs, use this direct link:
Note, you are NOT able to view usage/billing in Support Portal after migration to Google Maps Platform.
View billing, usage, and manage other resources and security for API keys in Cloud Console.