OneSignal is a high volume and reliable push notification service for websites and mobile applications. To know more about one-signal follow this link - https://onesignal.com/
Follow link to create an iOS APNs Certificate https://support.magplus.com/hc/en-us/articles/203808718-iOS-Creating-a-Push-Notification-Certificate
Follow this link to configure one-signal in your app https://documentation.onesignal.com/docs/ios-sdk-setup
Place your OneSignal App ID
Go to the app > project > Others > GlobalConstant.swift > ONESIGNALAPPID
Replace ONESIGNALAPPID = " Replace one-signal app id"