The text
<!-- Put your content below. Make sure to sign your edits with four tildes. ~~~~ -->
is part of the preload when a new Forum: post is created, but I think it would help a bit if the signature button was added to the JavaScript edit toolbar.

$wgExtraSignatureNamespaces = array( NS_FORUM );
should do the trick and be preferable to doing it at page load with site JavaScript.