Android Push-notification configurations
A) Open Firebase console, login to Firebase console.
B) Navigate to your project in which you want to add notification services.
C) Open project setting by clicking on setting button.
D) It will redirect to the setting of your project. In that click on cloud messaging tab, it will navigate to messaging service of Firebase.
E) Copy Legacy server key and add it to the plugin of the application store.
F) Open your store’s admin panel and redirect to App Settings/API Settings.
G) Then click on Push Notification tab.
H) Paste Legacy Server key in Android Legacy server key.
I) Then click on update button to save changes. Now you are done with the push notification services.
For more details documentation on Push notification click here