I have reviewed the office-js docs and not found any formal way of working with the clipboard through a Word add-in.
I attempted to use the newer navigator.clipboard.writeText but it is automatically rejected by the internal Word browser (where the add-in is hosted) without prompting the user (like a browser does). I have not seen any associated support for permissions policy that can be used to communicate with Word that the add-in would like clipboard permissions.
So, I am resorting to the deprecated document.executeCommand('copy') for now, but am concerned that will get my add-in rejected by the MS App Store.
Is there an official/sanctioned way to use the clipboard from within a MS Word add-in?