final FormPanel form = new FormPanel();
form.setEncoding(FormPanel.ENCODING_MULTIPART);
form.setMethod(FormPanel.METHOD_POST);

TextBox userid = new TextBox();
userid.setName("userid");
form.add(userid);

form.add(new Button("Submit", new ClickListener()
{
    public void onClick(Widget sender)
    {
        form.submit();
    }
}));

form.setAction("newPage.html");
RootPanel.get("demo").add(form);