The first step is to copy the page or post number that you want to link to.  Just select and copy from the ? onwards, so in this case
?page_id=279

Now go back to the page you are linking from.
In some cases you may need to remove some pre-existing underline or other formatting.  To do this, select the text and then hit the erasor. This will remove any formatting.

  Now select the text you want to make into a link, and hit the chain icon.

Paste in the earlier copied text, and insert the link.