How to add a prefix to dozens of sub-folder names

[JCLHfx]

I have 50 folders like this:
\Lossless\folder 1
\Lossless\folder 2
...
\Lossless\folder 50
I'd like to rename them to:
\Lossless\_F folder 1
\Lossless\_F folder 2
...
\Lossless\_F folder 50
(adding the prefix "_F " to a folder name helps me quickly identify folders with flac files)
I've tried to do that by adding "_F " to the prefix box in (7) and in filters (12), the mask * and folders ticked while files are unticked (I don't want individual files to have this prefix, only the folder names). I've also tried ticking Sub-folders to no avail.
When I click "preview" i get "There are no selected files that need editing".
What am I doing wrong?

[therube]

12:Filters, enable Folders & Subfolders

Select your \Lossless directory.

7:Add -> Prefix: _F<sp>

Make sure you've selected the 50 folders. New Name, should show the revised names...