We would like to create a link from some text in the Category Description. which opens Create Topic dialogue page (Just as the Create new Topic text link does.)
You seem to be on the right track. When I substitute my site’s domain into the template you have in that topic, it opens the composer in the correct category.
The problem is that Discourse want to automatically onebox the message link. You can stop a onebox from being created by either putting a space in front of the link, or by creating the link by clicking on composer’s link () modal.
You can also stop a one of by putting any text after the link. The link must be on a line by itself with nothing else in front or behind it, to one box.