You can't is the short answer. The browser is the only thing that can open up a new window.
What you can do is send a chunk of html down the response that has a link with your url as an href, target="_blank" and a chunk of javascript onload of the form that fakes a click. If this doesn't work then use a window.open(url);
response.write("<script>");
response.write("window.open('page.html','_blank')");
response.write("</script>");
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