PI SFTP adapter with variable substitution

All,

My intent is to add a "-" between the file name and the time stamp.  There is a SAP note that removed the hyphen from happening automatically.

I cannot find the available entries for SFTP for the circled field.  Any clues?

The documentation I found discusses the Reference entries like message:interface_name but the documentation does not explain what interface_name is.

Here is the Destination tab.  I have tried various approaches without luck.

Any ideas?

Thank you.