Go to the link located at the top of your browser. Click on the text and then on your keyboard press CTRL+A at the same time. This should select all the text in the address bar.
Next step is to copy the text. So now press CTRL+C which is the Copy command.
Last, you need to now reply to a post. In the reply area where you type the message, press CTRL+V which is the Paste command. Now the link should be there.