viewtopic is a known local pointer into the HST site. You simply open another browser window to the topic you wish to cross-post, copy the link, and just paste it back into your post in the original window. the HST bbcode makes it the short version. eg, I just copied the URL for this topic from another HST browser window and pasted here: viewtopic.php?f=2&p=45232#p45232
You can see it's handy-work by looking at the preview page of your post.
URL still works according to Eric, but the button returns when he chooses.