bittrette - can you please be more specific? Do you mean you want to add a link within a Forum post? For that you highlight the text you want to contain the link then click the link icon in the toolbar above the text (it is the 3rd icon from the right). Enter the link in Link URL and then also enter Open in new window in the Target field.
If you meant you want to post a link elsewhere on the site, please mention where and we will try to help you further.