Helpful Information

• You can test Firebase Cloud Messaging using this test application*. Push notifications do not require account registration.
• You can add an account through the system settings. Some applications might ask you to do so, if you don't.
• Apps that use Cloud Messaging must be installed after GmsCore, or else they will not work. Some applications that can run with microG GmsCore is installed in the correct order: TextSecure/Signal, Play Music, YouTube
• If you are using AdAway, make sure to put mtalk.google.com on your whitelist, or else problems are likely to occur when using Google Cloud Messaging. Thanks @benstyle1 on XDA for the hint.
• If your device is having trouble registering with Google Cloud Messaging, you may need to open the system phone app and dial *#*#2432546#*#* (or *#*#CHECKIN#*#*) to manually register the device as described here.
• alternative: (if typing via the keypad does not work.) execute as root: adb shell am broadcast -a android.provider.Telephony.SECRET_CODE -d android_secret_code://2432546 (https://github.com/microg/android_packages_apps_GmsCore/issues/660)

*possibly inaccurate if not up-to-date; if you're having issues considering testing with a more up-to-date application