You need Firebase account and onesignal account for documentation you can get information from link 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 Bussiness Settings > Notification