OTDS Authentication via web services

We are using sample code from https://knowledge.opentext.com/knowledge/cs.dll/Open/45500683 to authenticate OTDS users, and it appears to be working:

{code}

AuthenticationClient fAuthServiceOTDS = new AuthenticationClient();

OTAuthentication fAuthenticationOTDS = new OTAuthentication();

fAuthServiceOTDS.Endpoint.Address = new EndpointAddress(@"http://<server>:8090/otdsws/services/Authentication?wsdl");

string otdsToken = fAuthServiceOTDS.Authenticate(ref fAuthenticationOTDS, "<username>", "<password>");

{code}

However, I'd like to know if we have to provide a username and password, and use the currently logged on user instead? We do have single sign on configured, which is why I'd like it to use the currently logged on user instead of specifying a username and password.

I tried sending empty strings and null as the username and password but to no avail.