On the Premium Plan, you could authenticate using one of two mechanisms:
API key or Client ID + Digital Signature
To manage authorization (URL whitelisting), visit Support Portal. After a Premium Plan project has migrated to Maps Platform, many of the menu options disappear. To manage your Client ID on Maps Platform, you will need 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.