There is no built-in mechanism to pass a username/password on the URL as doing so gives "bad people" a super-duper-easy way to hack into Tableau Server itself. As a hacker, all I'd have to do is "sit on the wire", watch requests go to Tableau, and I could harvest everyone's usernames and passwords. Scary stuff!
You might want to read up on Tableau Server's ability to do Trusted Tickets authentication. You could essentially tell Tableau Server to "Trust" whatever other security mechanism is authenticating your users (I assume you have one). If you don't have another mechaism to authenticate users before they get to Tableau Server, there's not too much you can do.