Hi Dom,
To post a link to a specific thread, it is best to open that thread in a new browser tab. To link to a specific post in this thread, click on the message title of that post so that the post appears at the top. Then click in the URL bar of the browser, make sure that the entire URL is selected and copy this URL to the clipboard (e.g. with Ctrl+C).
Then go to your current post and paste the copied URL there (e.g. with Ctrl+V or right click > Paste). By clicking on the Preview button you can check whether the inserted URL has been correctly converted into a clickable link.
Hope this helps!
Best regards,
Chris