Hi @ferdinand ,
Thanks a lot, that's exactly what I was looking for !

flags: c4d.BaseContainer = c4d.BaseContainer()
        flags[c4d.FILENAME_DIRECTORY] = True
        # Add a CUSTOMGUI_FILENAME with our flags settings.
        self.AddCustomGui(id=MyDialog.ID_FILENAME, 
                          pluginid=c4d.CUSTOMGUI_FILENAME, 
                          name="Path", 
                          flags=c4d.BFH_SCALEFIT, 
                          minw=0, minh=0, 
                          customdata=flags)

Thanks @ferdinand,
Your post helped me a lot.
Any idea how I could set a DIRECTORY selector instead of a FILE selector ?

What is the corresponding pluginId to relace c4d.CUSTOMGUI_FILENAME ?

I know how to popup a directory selector with this command

c4d.storage.LoadDialog(c4d.FILESELECTTYPE_ANYTHING, "Select Directory", c4d.FILESELECT_DIRECTORY)

But how could I integrate this into a layout ?

Thanks

[edit @ferdinand]: Forked from https://developers.maxon.net/forum/topic/13872/