emacsbug: how to Cc: from a webmail client

[Juanma Barranquero]

admin/notes/bugtracker says:

** When reporting a bug, to send a Cc to another address
(e.g. address@hidden), do not just use a Cc: header.
Instead, use "X-Debbugs-CC:".  This ensures the Cc address will get a
mail with the bug report number in.

What to do when you're using a webmail client (Gmail) that doesn't
allow setting new headers? Can this be done as a pseudo-header
somehow?

 Juanma