If the user is allowed to select the installation path, Thinstall uses the Windows standard directory selection dialog provided through SHBrowseForFolder.
The system folder selection dialog will appear slightly differently depending on which version of Shell32.dll is installed.
Version 4.71 supports Edit box and the "Make New Folder" button:
Early versions of Windows which have not installed software components which update Shell32.dll will display a dailog looking like this:
To force the subdirectory to be appended to the user's selected path, visit this link.