Hi,
I would say you are correct. Custom HTML and create old fashion html a la:
<a href="http://www.yourexternalsite.com" target="_blank">Click me to go external</a>
Of-course this when using the html editor and not WYSIWYG editor, when using WYSIWYG, then you can just write "Click me to go external" and use the link icon to link that text to your external page.