You can also highlight a few appropriate words in your text, then click on add link and you will see the highlighted text appear in the second window copy and paste the URL of the page, post or site you are linking; hope this helps. If this is not clear you can go to the window at the top of your login page where it says- search for help here and type how to add a link to a page or post and a relevant training should appear.

If you're coping a raw link - you need to switch your editor from "visual" to the "text" mode - you'll see the tab at the top right corner of your page/post editor.