Finding form field for this html

I can't seem to find the form field or even form name for this particular
page, the relevant html is:

<form method="post" action="/cgi-bin/add_posting.pl" name="add_posting">
<input type="hidden" name="changedImages" value="">
<input type="hidden" name="pid" value="4112">
<input type="hidden" name="posting_cat" value="3749">
<input type="hidden" name="action" value="preview_or_images">
<input type="hidden" name="site_id" value="79">
<input type="hidden" name="posting_id" value="">

I have tried form(1); and it states there is no such form, I have also tried
form("add_posting.pl") and it said no such form "add_posting", the url to
the site is http://hull.gumtree.com/cgi-bin/add_posting.pl?posting_cat=3749


.