Change directory on a second directory page

Hi,

I have a problem with a second directory page. This page is for backing up an existing folder. With MUI_INNERDIALOG_TEXT I'm setting the backup directory to be displayed. This works fine.
After clicking Browse button and choosing a different location the application name from my installer is appended as subdirectory. There I don't want the application name to be displayed but the name of the backup directory. How can I set this to be displayed after browsing???

Function DirectoryBackShow
    !insertmacro MUI_INNERDIALOG_TEXT 1019 "$BACKUP_DIR"
FunctionEnd 

I now figured out that InstallDir makes the substring following the last \ to be appended when Browse was selected.

I think that works for pages using the "original" INSTDIR variable. But I don't know how to do this for a second directory page which is using a different directory variable by giving

!define MUI_DIRECTORYPAGE_VARIABLE "$BACKUP_DIR" 

Can somebody help me please?!