How do I get an API key to use Firebase Cloud Messaging in SNS now that the API has been deprecated?

When creating a mobile application in SNS for the Firebase Cloud Messaging platform, the form asks for the 'Firebase Cloud Messaging Credentials', in the form of an API Key.

The 'Cloud Messaging API' in Firebase has been deprecated, the 'Firebase Cloud Messaging API' is now the only available API. This API requires the setup of 'Service Accounts' which to me do not appear to have the ability to create an application API key.

How do I get an API key that will work in SNS or send push notifications to Android devices through SNS?

Thanks.