How do I make a web extension that sends PSA announcements to other users?

Basically, what I'd like to make is a simple pop-up window browser extension that lets me enter a text message into it, and that message then gets sent via the internet to other users who have the extension installed, and appears in their web browser as a popup that can then be closed.

I know the basics of extensions, but I am not sure what sort of services I'd need to accomplish this message-sending over the internet. I also wouldn't mind all previous messages being saved. Are there any free APIs I can use that will help me accomplish this?