For example, I wish to perform Windows+E to open the Windows file explorer in the Windows environment running over Citrix from my Mac. The Citrix-Online-Plugin-for-Macintosh-Administrators-Guide  talks about other special keys but says nothing about the Windows key. I was somewhat surprised that google searches didn't readily identify any discussion about this topic. Perhaps I just don't have the right keywords?
Specifically, the right-hand Command key works for me. The left-hand one seems to simply emulate the Ctrl key.