How to Insert as Text for Outlook 365 online version

I need to provide the option to use HTML for email to clients.

I would like to mimic the old "Attach -> Insert as Text" functionality in the online version of Outlook 365.

I've seen solutions for the desktop app, where you simply show the "old" button in the ribbon. But the only version doesn't have (or I don't know where it is) a customizable ribbon that will allow that.

Thanks! Wayne