To see in action. Follow the steps :

• Click on subscribe button. On the bottom right of the page.
• You will be asked to allow / block notification. Click on "allow".
• You would now see a bell icon replacing the "subscribe" button
• Now click the bell icon.
• A popover would appear with the subscription object. Copy it all.
• Open this link
• Paste in the "Subscription to Send To" section
• Type in your message you want
• Now click "Send message" and see a popup should come.