Simply open NSUserNotificationPrivate.xcodeproj and run.

Have a look around AppDelegate.swift to see what properties are being used on the NSUserNotification class

In the NSUserNotificationPrivate-Bridging-Header.h (Foundation.framework class-dump header file) file, you will find other properties that can be used on the NSUserNotification class:

@property(copy) NSImage *_identityImage;
@property BOOL _clearable;
@property unsigned long long _badgeCount;

To set _identityImage, or any other private property, call the instance method setValue:

notification.setValue(NSImage(named: NSImage.Name(rawValue: "img")), forKey: "_identityImage")
notification.setValue(true, forKey: "_clearable")
notification.setValue(5, forKey: "_badgeCount")

The following properties are used to present a notification as shown in the image below:

_identityImage _identityImageHasBorder _ignoresDoNotDisturb _lockscreenOnly _poofsOnCancel _alternateActionButtonTitles _alwaysShowAlternateActionMenu _clearable

In order for a notification to be able to have action buttons (buttons on the right hand side), the following key value pair must be added to Info.plist:

Key: NSUserNotificationAlertStyle Value: alert