I think you are trying to login to a website from your page, without showing login page of the website.
You can do this by submitting the form data using POST method.
you will have to open the login page of the 3rd party website using 'View Source' option in Internet Explorer, to see the HTML code for the page.
Now grab the From name, and User Name / Password field text box name from there.
and then pass all these 3 values using the POST method.
You can find an example on who to use it here.