Go to the page you want to link to, right click and copy it's web adress (URL).
Start your forum post -
Highlight the word you want to be the link in your post.
In the bar at the top you'll see the chain link icons have lit up.
Click on the left one and a tab will open up like this-
Where it says Link URL,right click and paste the link's URL in there.
Click on the wee arrow in Target box and set to 'open in new tab.'
Ignore the 3rd box 'Title'.
Then click on insert and hey presto the word you highlighted becomes the link.