Why is it not possible to see the Remote Desktop extension in Studio?
The Remote desktop extension was released with the 2019 version of Studio ( specifically 2019.4 Enterprise version).
If the current setup is on UiPath 2018.4, the Remoteruntime installer would still be available to be used with the Citrix extension, however for the Remote desktop extension will not be available.
In order for the same to be used, the Studio will have to be updated to 2019 or above. More details on the RDP extension is available at https://docs.uipath.com/releasenotes/docs/2019-4-2#section-rdp-extension .