Posting Data With Window.open?

I have an application with a button that opens up another website. That website requires credentials. I'm currently using window.open to get the url open and passing a variable to the end of the URL, but when I have searched for a good way to get the credentials passed by using post and can't seem to find a solution. I tried xmlhttprequest, but the limitation of cross-domains is huge because I will have external clients connecting to the database and don't think that asking them to lower the security settings in IE would be a good way to do it. All I need is the ability to click on the button, pass the username and password and have the url open to desired page. I posted the code of the application in another thread, but all I'm getting are just views.