How do I authenticate to a z39.50 server using yaz-client?

There is a ProQuest database/index I want to search via their z39.50 interface. I have a valid username/password combination that I can sucessfully use to configure and search with my EndNote z39.50 client. How do I supply a similar configuration to yaz-client? I see yaz-client supports a -u switch, but what form does the -u parameter take?

— 
Eric Morgan