BW> I would guess at something like (for entering "bbs" username at the
 BW> mgetty login prompt, executing Mystic as "bbs" user, and logging it as
 BW> user "bbs" as well):
 BW> 
 BW> bbs bbs bbs /home/bbs/mystic -ST3 -IP127.0.0.1 -HOSTlocalhost 

You probably wouldn't want to set an IP and hostname like that although it may not hurt anything.  Its intended to repurpose those fields with caller ID information so for example you might replace IP with the phone number and HOST with "Serial Modem" or something.

But I think from the looks of it you're on the right path.

... How is it possible to have a civil war?

--- Mystic BBS v1.12 A47 2021/08/17 (Windows/64)