When you get a new desktop notification, your browser will ask for your permission to show the desktop notification by default.
Click the "Allow" button in the pop up
Select "Always Show Notifications"
You can always change the notification permissions for a site.
The web notification is supported by Firefox, Chrome, Safari and Opera very well, however, IE and edge don't support it. (Our notification will be shown in the top right corner of your browser if your browser doesn't support web notification or you don't give permission to show as desktop notification)