Follow below steps to get Firebase Server Key :
Step 1 : Go to Firebase
Step 2 : If you have a Firebase account, Sign in, else create an account
Step 3 : Click on Get Started
Step 4 : Select your project
Step 5 : Click on cog icon
Step 6 : Click on Project settings.
Step 7 : Click on Cloud Messaging
Step 8 : Under Project Credentials, you will find the Firebase Server Key (FCM Secret key). Copy the key to your clipboard.
Note: The Firebase Server Key and FCM Secret Key are the same.