MrClaus - Every forum uses a different system of coding for links and images, it's not too hard to learn them.
On this forum when you have the "new thread" or "reply" webpage open, you'll see a number of small icons across the top of the text section.
If you place your cursor on the icon with the blue globe and chain link below it, the globe icon will brighten and you'll get an "insert link" message box appear.
Left-clicking on the blue globe icon produces a pop-up message box as shown below.
If you have copied the URL to your clipboard, merely place your cursor in the pop-up message box and press CTRL and V simultaneously to paste the URL into the pop-up box.
Make sure the "http" part isn't duplicated. Then click "OK" and the working link is automatically added to your reply.
You can check to see if the link is actually working properly by clicking "preview post", and the link should show up as a highlighted and working link.
