The "Leave us a private message" link is currently in the footer of the widget, which makes it hard to find. It should at the bottom of the input box like it is on the forum home page to be consistent.
Also, consider changing the workflow such that it becomes an option next to the final Post button. I.e. have a "Post to forum" and "Send private message" button. This way, private messages also go through the "search for matching topic" process which might reduce unnecessary messages.