Hi Sagi,
You can use a code inside typology rule
delivery.mailParameters.headers inside the JS code will give you all the headers.
If you do a split
var headerLines = headers.split("\n");
you get an array of headers.
Create your custom header as
var newHeaderLine = header + ": " + value;
Add the new header line to the array
headerLines.push(newHeaderLine);
Finally, join them and assign to the main parameter
delivery.mailParameters.headers = headerLines.join("\n")
Regards,
Vipul