Hyperlinks can be added to the Highlights section of listings, simply by using standard HTML tags. This displays correctly (and the links will work) from the ShowMojo schedule-a-showing pages.
Obviously, be sure to visually and functionally test any links you add.
IMPORTANT: The hyperlinks don't work when ShowMojo syndicates to listing sites like HotPads, Zillow and Homes.com. Only "visible text" will appear on these site. The hyperlinks will not appear and will not work. So be mindful to avoid using phrases like: "Click Here".