Form post with URL paramters (after question mark)

I had a URL that I wanted to POST with JavaScript so a made a

        <form name="doForm" action="" method="post">
        </form>

and used JavaScript to set the action and call submit. But Tomcat showed the request as a GET. I found that adding a dummy form input fixed the problem:

        <form name="doForm" action="" method="post">
            <input type="hidden" name="blah" value="blah"/><!-- need at least 1 input or the form uses method="get" even when "post" is specified. -->
        </form>