To add a link to another location on the same page, you will first need to create a named anchor on that page. Anchors point to a specific place within the page.
Select the appropriate location (e.g., a heading) and click the "Insert/Edit Anchor" button in the toolbar (you will have to expand the toolbar to see the additional buttons).
In the "Insert/Edit Anchor" dialog box, enter the name of your anchor.
This will create the named anchor (indicated by a yellow anchor symbol).
To link to your newly-created named anchor, highlight the text, then click the Linkit button. In the "Link URL" field, add a "#" symbol and your anchor name:
Click "Insert link" to add the link to your named anchor (on the same page).