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I began to explore the terrain for a Word plugin for the company I’m working.

• Although there is a common format for the manifest file, Outlook and Word addins seems to be required to have it separate. I didn’t see any addin in AppSource that’s both Word and Excel addins.
• There is a tool from Microsoft called Script Lab that allows to run arbitrary scripts and testAPIs in Office.
• For authentication, it’s better to open a browser window to your web app via Dialog API
• It’s usually not possible to use iframes to get authentication tokens from the taskbar/sidebar itself. See this answer.
• It’s possible to add elements to right click menu in Word for add-in commands.