If you want the Editor enabled, go to your Account settings and click the option ‘Enable editor’. This will activate the WYSIWYG (What You See is What You Get) text editor throughout the system for you.
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This will allow you to edit with the HTML Editor in the system. You will be able to have the bar below and see the text as it would be viewed.
Alternatively, if you want to see everything in HTML, you can disable the WYSIWYG Editor and you will be able to edit the text invites with HTML script. Alternatively, if you have the Editor enabled, you can click the <> at the top left, and it will take you to the script page.