From the SSH protocol's point-of-view, it is actually impossible to attempt authentication without specifying a username. Therefore, of course, Rebex libraries do support this - otherwise they would not work at all.
If you do "ssh server" with OpenSSH's client, it simply uses the current user's username and sends that to the server. You can easily see this if you enable debug log: "ssh -v server".
With Rebex API, you specify the username in the Login method. Even the parameter-less Login method overload invokes AuthenticationRequest first to ask for the username.