WinSCP Commandline Private key

What I am trying to so is: Open a WInSCP-Session using a private key instead of a password.

The simple Password-Approach works:

 WinSCP.exe root:[email protected]

But the next step, using the key, does not work. Just to be clear. I mean this key:

I tried:

 WinSCP.exe root:[email protected] /privatekey "path/to/private_key.ppk"

But WinSCP ignores the parameter and asks for the password. So, what did I forget?