The "Desktop Sharing Screen" and the accompanying "Desktop Audio Speaker" props can be used on PC systems in private Custom Rooms to share your desktop screen and audio to other players on any platform. Note that the shared screen is updated at 1 frame per second, which is enough for slide shows, web browsing, and editing documents, but not for watching movies.
These props are available in the "Props" tab of the Palette (in the "Creative" category) after you have successfully applied to email@example.com with your @-username and your reason for sharing your desktop.
Update: Rec room is no longer accepting emails asking for the screen they tell you to go to (http://recroom.happyfox.com/new) and ask them only if you are an educator or in a company that wants to use it for presentation like things.
Also if you had the screen its most likely that rec room removed it form your pallet and if you try to use one in another room it will say "desktop sharing is not supported."