Passing custom headers to login page

I am working on the ability for a site (A) to open another site (B) and
automatically log the user in.
The two sites will not be on the same server.

I am using Forms based authentication.

I have this working with query strings.  I grab the user name and password
from the URL, decode them, put them in hidden fields, and then use
Javascript to populate the fields and click the login button.

I'd rather not use the query string parameters to pass the user name and
password.  I'd prefer to pass the values in custom headers but I can't get
that to work.

I've used Response.AddHeader() in site A both before and after my call to
Response.Redirect() but no matter what I do my custom headers are not in the
Request.Headers in site B.

My code looks something like:

        protected void Open_Click(object sender, EventArgs e)
        {
            Response.AddHeader("Test","MyTest");
            Response.Redirect("http://localhost:4723/SiteB/Login.aspx);
        }

This is a call directly to the Login.aspx page so it has not been
re-directed from another page for authentication (which would cause the
headers to be lost).

Does anyone know how I can do this?

Thanks,
-Mont

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