authenticated reverse proxy

I am looking for the ability to be able to have a hyperlink on my website that goes to a password protected page where a user will put in their user name and password. From there the site will be directed to another site that will automatically start a new session and fill in a master password and username. this second step needs to happen in the background unknown to the user as to the master password and log in name. To the user the whole process will look seemless. this needs to be completed ASAP