[Users] Filtering by Envelope-to

[Adrian]

I have a ton of spam going to my catchall user, that is sent to a
username spam-harvested twenty years ago.  Of course, being spam I'm
blind-copied.  I include the Envelope-To in my headers - is there any
way I can add Envelope-to as a header field in a new filter rule?

The user-contributed FAQs suggest you can add headers.  I can see the
dropdown of header names but I can't see where or how to add to it.
Using ver 4.2.0.

Thanks!