You enable push notification using the Server app.
When you enable push notification, you need an Apple ID associated with your organization. Using a personal Apple ID isn’t recommended. You must have, or be ready to create, an Apple ID before you can turn on the service.
Select your server in the Server app sidebar, then click Settings.
Select “Enable Apple push notifications.”
Enter the Apple ID and password.
If you don’t have an Apple ID for your organization, click the link to create one.
Click “Get certificate.”