NOTE: The techniques outlined below require a lot of switching of web pages, so it's worthwhile knowing that in Windows a "wheel-click" opens a link in a new tab, rather than replacing the contents of the current tab with the new page. Having a number of tabs open makes switching between pages that are to be the target of links and embeddings so much simpler! If you just left-click links then you are likely to lose anything you type on a page by the time you return to it!
If you know you will want to link to an existing topic then, before starting the message...
1. Visit the page you wish to link to - such as this one:

2. Highlight the URL in the Address Bar

3. Hit CTRL+C (or use whatever method you find easiest to copy a selection to the clipboard)
4. Switch to message you are writing where the link is to appear
5. Select the word or words you wish to form a link
6. Click the LINK button at the top of the EDIT box

7. In the box that appears on the greyed out window (Don't remove the selection on the existing "http://") hit CTRL-V (or your preferred method for pasting clipboard content) to replace the current selection with the full URL you placed on the clipboard.

Job Done!