You need Firebase account and Onesignal account setup, you can get information from official links below.
First of all you need to create a project on FCM then add Firebase to your account to get sender ID using URL below
You can get sender ID after that you can create app on one signal to get App ID using link below
Now avail this feature, put both app ID and Sender ID at back-end make this feature functional.
To enter your Onesignal Credentials use below link. Go To Business Settings > Notification
