Freron MailMate

I need to add a custom header to a sent message. The documentation at https://manual.mailmate-app.com/customization#:~:text=x%2Dnewsreade... suggests that there is an "Others…" option in the headers popup.

The ticket system doesn't let me paste a screenshot, unlike GitHub, but this is the menu I see:

From
To
Cc
Bcc
Reply-To
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Send Later
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Use as Default Headers

How do I add a custom header if the method given in the documentation isn't available?

I only need this for ONE message. I don't want to change my default headers for every message. I don't want to quit MailMate, edit a plist to add the header to that list, start MailMate, send the message, quit MailMate, remove the header from the plist, and restart MailMate again because that's a huge hassle :)

Was the Others… option removed? Is there any chance of it returning, please? (Or is there a way to do it already, but the documentation is out-of-date?)