You may have solved your challenge but if not, you should have a look at DTech Apps Docintegrator.  DTech Apps DocIntegrator simplifies sending attachments to a third-party SharePoint URL in Flow Designer by providing a pre-built "Upload File to SharePoint" (or similar) action. You configure this action with a secure DocIntegrator connection to the third-party's SharePoint, specify the SharePoint URL (stored directly in the action, a CI, or a system property), the desired file path, and the attachment content (potentially needing Base64 to binary conversion). This eliminates complex API calls and certificate management, offering a straightforward way to integrate with external SharePoint environments.