Flash Forms and hidden fields

I’ve built a form in Flash that’s a basic username and password form. I’m trying to send two additional variables with that form, simulating hidden form fields in HTML.

I tried creating a variable to hold the string and then pass it through the function like this:

[AS] var StartURL:String= “/cgi-win/auth/$campredir.dll/MainPage”;

// Provide custom form submission function.
function submitForm () {
sender.UserID = userName_txt.text;
sender.Password = password_txt.text;
sender.StartURL = StartURL;
sender.send("/cgi-win/$login.dll", “POST”);
}

[/AS]

Is this possible? What am I doing wrong?