I need to link to an external URL from a slider that pulls in The Events Calendar events. You can see it in action on the home page just above the footer. http://kitchener.faithfm.org/
I can see you are using the tribe_get_organizer() function to print the href attribute. That function will return the organizer name.
Try using the tribe_get_organizer_website_url() helper function instead.
You can find out all the available Organizer Functions for use in the templates in the organizer.php file located at /wp-content/plugins/the-events-calendar/src/functions/template-tags/
I hope that helps! Let me know if you have other questions.