If you mean the root of the folder structure on the server, then you can refer to it as simply $/
. However, if you mean the local folder that this is mapped to, there's not a macro for that. The reason is that you can map any number of folders in the server hierarchy to local disk folders, so there's not necessarily a single root - and there's not necessarily a folder that maps to $/
at all.
(That said, if you can explain why you need it, there might be alternative suggestions...)