Yes, you can. You can go to your event editor, then in "Calls to Action", while you are configuring your call to action, click on "<html>", and a html code editor will open up. You can add any valid html code there.
Once you add the code, you can edit that html block by taking your mouse on the html content and hitting on the pencil icon.
Please Note: Javascript codes will not work, only html codes will work when you use the html editor.