Public key/Private key authentication

I am trying to use the WinSCP client to FTP to a server that requires me to provide a public/private key pair. I've searched the documentation but can't find any references to this authentication. Is it possible with the client?

Yes, see https://winscp.net/eng/docs/guide_public_key#winscp

Or do you want to use it from code? You didn't mention that, but as you are posting this to the .NET assembly forum: Use SessionOptions.SshPrivateKeyPath:
https://winscp.net/eng/docs/library_sessionoptions#sshprivatekeypath