I'm using 4.12 beta on XP.
When I select a directory the name of which begins with
"-" (hyphen) e. g. : -mydir and clilck on Add button,
the following message appears:
"Incorrect command line"
and the commpression does not start.
Logged In: YES
p7zip don't provide an "add button".
I think that you have used the wrong tracker.
You must use the tracker of 7-zip for Windows :