Here's one zbell implementation, not sure it's the original but it looks like it does the trick: https://gist.github.com/oknowton/8346801
I developed the tool myself, and it's at https://git.sr.ht/~bayindirh/nudge if you feel like checking it out.
If you want to host the whole push notification infrastructure, you can look at https://ntfy.sh which also can be integrated with cURL.
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Here's one zbell implementation, not sure it's the original but it looks like it does the trick: https://gist.github.com/oknowton/8346801