Notify icon control is readily available in WPF. Its implementation in the back end code is also pretty simple.
But this Notify icon is suitable only to display short and crisp messages. Initially I too had implemented the same for my project. You can also write the required code in the events.
But the problem came when the client asked for more than single line message. They expected us to add a button, checkbox and image. Well initially I thought it is simple to add those. When browsed through the net, I came to know that Notify icon has its own constraints. It has limitation in the message length and anything you need to add, you add as list to the tray menu which is available on the right click of the Notify icon which was not meeting the client requirement.
Then I came know the same can achieved by using User control. Using User control you can add anything, it is just another xaml page. With UI styling and control properties available you can make it look like Notify icon itself. So I would suggest to use the control which is helpful in implementing many things than Notify icon, if your requirement is more in terms of display.
This window can be called in required code as pop up