SSHNet SFTP and OpenSSH and shared directories

[holzert]

Running OpenSSH on Windows Server 2016. Using Renci.SSHNet as sftp client. Trying to use SFTP to transfer files to a directory share on the OpenSSH server.
ChrootDirectory /FileDrop

doesn't work.
Nor does //FileDrop
Nor does /
and then using SFTP to change to /FileDrop or //FileDrop

How to use a mapped network directory e.g. c:\AllDroppedFiles shared as /FileDrop ? OpenSSH service will not even start when ChrootDirectory in sshd_config file is set to /FileDrop or //FileDrop etc
Only paths I can get to work seem to be true paths on a drive e.g. c:\AllDroppedFiles will work.