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

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. link3

  Now select the text you want to make into a link, and hit the chain icon. link3

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