The GUI application (Windows) has an option on the "Browser ID" tab for
"Additional HTTP Headers", and you can add headers to this box that get sent
on every HTTP request.

In the CLI documentation though (https://www.httrack.com/html/fcguide.html),
there's no corresponding option that I can find (same issue for the "Default
Referrer URL" field in the GUI).

Is there any way to set additional HTTP headers with the CLI?