I'm looking to link text in the pre header - for example - "Register now". I'm trying to do this in the pre header area under email settings and not by altering the html. We are working on new email templates which will allow us to edit the pre header directly in the editor. But in the meantime looking to see what formatting options are available using the native functionality.
Don't do this. No recipient can click on such a pre-header text, which by definition is just text that appears in the preview in some clients. You only get 60-80 char anyway.