You can insert links to external (third party) websites into your site navigate by using a Redirect page type.
In Page Manager, create a new page, select the Page Type "Redirect," and save the page. Be sure to enter a name for the page in the "Page Name" field, as this is what will appear in your site's navigation.
The "Redirect Properties" section will appear in Page Manager. Enter the address of the destination page in the "URL" field. Use the checkboxes to set whether the destination page will open in a new window, or the current browser window. (It's not uncommon to open external sites in a new browser.)
That's it! The new page will appear in your site's navigation, and when users click on it they'll be taken to the address you entered.