Assuming that the path this third-party application uses for this command line argument is relative to its "Skins" directory, the "In this working directory" field should likely be left blank.
Looking at
the documentation, that parameter takes a relative path to a directory, so if you have a folder structure like "Skins\deeper folder\a skin", the required command line argument would presumably be
!ActivateConfig "deeper folder\a skin"
None of this is VoiceAttack-specific either; you'd need the same syntax when using a desktop shortcut or a command prompt.