How to extract WSDL from authenticated service

I've just downloaded and installed SoapUI, and the first thing I need to do it get the WSDL from the web service I need to test. However, the service is authenticated. I have a username and password, and it appears that at some point I can enter these into SoapUI, but not until after it reads the WSDL - which of course it can't because of the authentication.

Any suggestions on how to do this would be appreciated.