6. Then you'll see this screen. Click on Google Android (GCM) and click NEXT.
7. Then on next step, you need to provide the Google Server API Key and Google Project Number. You'll find them in your Firebase Console. If you haven't create a firebase app for your project, then go here. Then click on save
8. Select the app you've already created in firebase console. Then you'll see something like this. Click on the small "GEAR Icon" and then click on Project settings.
9. Then you can find your Server Key and Project Number in the CLOUD MESSAGING tab.
10. After filling these information, click on Save. And follow the instructions in image to obtain your OneSignal App ID and OneSignal REST API KEY.