I receive a reply email from another ticketing system which contains an ID. I would like my reply to this message to contain this new and external ID in the subject. I tried to do this with a custom function that a user kindly shared here:

But even creating the fields with the same names as the example and replacing .eu with .com I don't get the result I was hoping for.

At this point, I would like to enter it by hand when I answer. Is it possible?