First, blue the web address you want to link to, then right click and click on copy. This will make the address go into your mouse (yes, it does! it must do) Then in the posting box type your two double square brackets, paste address from out of your mouse into your message, leave a space and type something like "this" (or whatever you would like to say) And then it's done.
If you are using a tablet of course, you copy with your finger, and so the web address actually goes into your finger. (yes, it does!)