The users are unable to map the Windows key (particularly relevant to use Windows 8 devices) in Mac Receiver versions prior to 11.8 clients.
Upgrade the Mac Receiver version to 11.8 clients to map the Windows key in the ICA session.
The feature of mapping the Windows key inside the ICA session is not available in Mac Receiver versions prior to 11.8 clients.
Citrix Documentation - About Receiver for Mac 11.8
“Windows shortcuts - Support for Windows logo key shortcuts is built into the Keyboard menu. This allows Mac users to send Windows shortcuts to their remote desktops and applications.”