I'm not aware of any way to use Microsoft AutoSave with iCloud. When I turn on AutoSave I get a message saying "To turn on AutoSave and access your document from anywhere, upload your file" and the options are to Microsoft OneDrive Personal or to other Microsoft cloud locations.
Also see the information in https://support.microsoft.com/en-us/office/what-is-autosave-6d6bd723-ebfd-4e40-b5f6-ae6e8088f7a5
including the excerpt below:
AutoSave is enabled when working on a file that is saved to OneDrive, OneDrive for Business, or SharePoint Online. If the file is saved to another location (or hasn't been saved at all, yet) then AutoSave is disabled. This will happen if your file is on a SharePoint On Premises site, a file server, or saved to a local folder on your computer.