In order to configure and enable push notifications for iOS apps, you need to use the Apple Developer Center to create mobile provisioning and push notifications configured, and obtain APNs certificate.
Note: An Apple Push Notification service (APNS) Certificate is required when using push notifications to send push messages from the server to iOS mobile device. Information on generating an APNS Certificate is available in the iOS Developer Library.
The single private key and a single APNS certificate must be saved in a P12 file so that it can be uploaded and saved in a certificate set for the iOS custom app you are creating - see Configuring push notifications on iOS.