The External url, button, IFrame widget advanced option allows you feature a link either as a button or an IFrame on an item.
To feature an external URL on an item:
Optionally, a link can be set to open as soon as practically possible, e.g. without first displaying any other content for this item. For example, by setting the External url, button, IFrame widget advanced option for an item called "Forum" under Home, clicking on "Forum" in the top navigation on your website may open a third party link to a forum immediately, without first displaying a website page called "Forum".
Even if the above is intended behavior, do not forget to add some content to such an item; there are many ways your audience (and search engines!) may find your 'forum' page, even if you ultimately intended for them to go to a different link.