I don't have a direct answer, but found this and thought it might help you: http://mkt.tableausoftware.com/downloads/embedding_views.pdf
Someone here may have actual experience and be able to help answer your question more directly.
Tableau doesn't support this, as passing a username/password on the URL is extremely insecure.
Please investigate the feature known as "Trusted Authentication". It will allow you to log-in users without them having to address a log-in screen. You can then use the "User Filters" feature to show/hide views based on their identity.
Hope this helps!
Tracy, thanks for the response. So let me clarify. I have an embedded workbook and in order for the workbook to only show what they need to see they would need to have their username tied to the data being displayed, correct? If so, then that would be a filter that is passed along when clicking through the link when capturing the login information, correct?
Tracy is right, you need User Filters + Trusted authentication.
Trusted Authentication: Trusted Authentication
You should use user filters in the embedded workbook/view and logon users from your application (where you embed your view) with trusted authentication. That is the standard way.
I am sure that you can't say this user/pass thing seriously Why you would store ALL of your users password in cleartext in your own application to pass to tableau? Make no sense, however, it is possible like everything in IT. But let just try the standard option
User Filters (yes, filters data according your user identity) and Trusted Authentication where you can logon your users without the need to pass their passwords. Trust me, this is exactly what you need.