You can take and move all files in the directory folder, when you edit the pseudo tagfield _DIRECTORY.
Try to "format value" the pseudo tag field _DIRECTORY instead of the pseudo tag field _FILENAME.
This will cause a MOVE of the complete folder.
When you load the track files, respectively the parent folder, respectively the root folder, into Mp3tag, you should have a closer look to the fact where is the Mp3tag workfolder situated (should be read in Mp3tag window title).
In relation to this workfolder you can assemble a relative folderpath which points to the place where you want to create the new subfolder of your wish.
As an alternative way you can use an absolute path starting at the drive root, carrying the complete folderpath which points to the new folder.
See also ...
/t/16792/1
Ordner erstellen und Dateien verschieben
DD.20170605.0838.CEST