Login or Signup in https://onesignal.com/
Add an app, Add you app name and click ADD APP
After that select platform and hit NEXT:
Next you need to get Firebase Server Key and Firebase Sender ID. You will get it with Following steps
In another browser tab Open Firebase Console https://console.firebase.google.com and sign in with your Google account.
Press "CREATE NEW PROJECT" or select an existing one below.
Enter a project name and press "CREATE PROJECT".
Go to project OverView
Click Settings -> Project Settings -> Cloud Messaging
Copy server key And sender Id
And Add it in OneSignal app and click SAVE
Select your target SDK PhoneGap, Cordova, ionic and hit NEXT
Copy Your App ID
Add in OneSignal APP ID, Firebase Server Key and Firebase Sender ID in Wordpress -> Mobile App -> Settings
After that Build apk and Subscribe at least one user by launching your app on mobile. Then you need to hit check subscribed user and click DONE