I was able to successfully connect the Windows machine with PowerShell/Win32-OpenSSH using the "SSH Build Agents Plugin for Jenkins" using the "Remote root directory" of "C:\jenkins-agent", and I'm able to run Windows builds on this agent.

The issue occurs when I change the "Remote root directory" to "D:\jenkins-agent", then the agent fails to launch:

[04/20/19 09:21:28] [SSH] Remote file system root D:\jenkins-agent does not exist. Will try to create it...

[04/20/19 09:21:28] [SSH] Copying latest remoting.jar...

[04/20/19 09:21:29] [SSH] Copied 789,283 bytes.
Expanded the channel window size to 4MB
[04/20/19 09:21:29] [SSH] Starting agent process: cd "D:\jenkins-agent" && "C:\Program Files (x86)\Java\jre1.8.0_201\bin\java.exe" -jar remoting.jar -workDir D:\jenkins-agent
Error: Unable to access jarfile remoting.jar
Slave JVM has terminated. Exit code=1
[04/20/19 09:21:29] Launch failed - cleaning up connection
[04/20/19 09:21:29] [SSH] Connection closed.

The root cause appears to be that the command 'cd "D:\jenkins-agent"' does not successfully change to the D:\ drive, because when using Windows command prompt, you need to add the '/d' argument to the 'cd' command to change the drive:

/d : Changes the current drive or the current directory for a drive.