Secure way to post a web form ... client side

[Darrell G]

I am in need of some good advice ...

Goal: I need to automatically log users of my website into
a second website.

Obstacles: The second website requires I "post" a form
with specific field/value combinations before it will
execute and return a valid cookie/session. The solution
also needs to be secure and as transparent as possible.

Solution (so far): I have created an asp.net web form I
populate with the login information and deliver to the
users browser. I then use JavaScript to immediately submit
the form (after it is loaded into Internet Explorer) to
the second server.

Problem: Although this solution works its not secure. If a
user can view the source of my page before it submits to
the second web server they will find the login info (not
acceptable).

Question: How can I automatically and securely "post" a
client-side form? Could I use something like an ActiveX
Control or a Java Applet to securely accomplish this? If
so, which one would be the best and or easiest of the two
to work with? Also, will the browser hosting them inherit
the cookies presented by the other website?

I am in unfamiliar territory here ... help please!

- Darrell G