Push Notifications can be used in most every service that macOS Server 5.4 (for High Sierra) can run. Any service that requiring Push Notifications will often provide the ability to setup APNS during the configuration of the service. But at this point, I usually just set up Push Notifications when I setup a new server. 
To enable Push Notifications for services, you’ll first need to have a valid AppleID. Once you have an AppleID, open the Server app and then click on the name of the server. Then click on the Settings screen and click on the checkbox for Notifications. 
At the Settings screen for your server, click on the check-box for Apple Push Notifications (APN). Next, click on another screen and then click back to get the Edit Apple ID… button to appear. Click on Edit Apple ID…
At the Apple Push Notification Services certificate screen, enter an AppleID if you have not yet configured APNS and click on OK. The Apple Push Notification Service certificate will then be configured. 
As you’ll see, if you’re editing a certificate, you’ll break any systems or services that use that certificate. For example, you would have to re-enroll all of your Profile Manager systems. Instead, use the Renew button whenever possible, prior to the expiration of certificates.
When renewing certificates, you’ll provide the SAME AppleID and Password you used to generate the original certificate.
The certificate is valid for one year, by default. Administrators receive an alert when the certificate is due to expire. If you don’t have the credentials for the AppleID used to obtain the original certificate you can’t renew; in that scenario, open the same screen and click on the Change button. Once you have generated a certificate, you’ll then be able to see the certificate in the Apple certificates portal, but you’ll have to re-enroll devices if using Profile Manager.
