It sounds like you're trying to use a feature from GCM that allowed you to use multiple API keys with a single project. That's not possible in Firebase Cloud Messaging (FCM).
An important detail to always include here is the use case; Why do you want to use multiple API keys? (see
XY problem).
If it is so that different senders can send to the app then it is better to have each sender create their own project then have the app register with all senders. This allows separation of responsibility in the event of throttling, one sender's bad behavior will not affect other senders.
If there is only a single sender (the developer) with multiple apps then there is no need to have multiple API keys, the apps can all use the same sender ID and API key.