In the WYSIWYG editor:
  1. Highlight the text that should serve as the link, then click the button that looks like a chain link and says "Insert/Edit URL or Link"
  2. Either click on "Browse Links" to find the page you want to link to, or manually enter in the URL in the "Link URL" area
  3. Under "Target", choose "Open link in a new window"
  4. Click on "Insert"