For example you can use pushnotify (seems outdated, but is working) and a small webserver that is handling the webhook from zammad.
Danger Zone
I’m not sure, if pushnotify is DSGVO-conform.
Create a webhook that is calling a php script, put your own payload in there or use the given one, and in your php script call the pushnotify api with a simple get request.
And the webhook calls a php script on my web server. The script simple contains the following:
<?php
$json_string=file_get_contents("php://input");
$data=json_decode($json_string);
$txt=urlencode("A new Ticket \"".$data->ticket->title."\" was created in group \"".$data->ticket->group_name."\" with ID ".$data->ticket->number);
$response = file_get_contents("https://pushnotify.co.uk/send/?userid=yourid&code=yourcode&txt=".$txt);
?>
This sample script has nether user authentication for data, which is coming from the webhook, nor any data validation so be careful when using it!