The add-in (manifest file) is cross-platform. It allows the end-users to see their email signatures as they compose their email. In addition, they can select among multiple email signatures (shared mailbox signatures, other languages, an internal signature, etc.). You can allow the end-users to customize their email signatures(s) in a controlled manner (for 100% flexibility – E.g. LinkedIn/Twitter profile links, tick mobile off, select the manager signature when sending on behalf of, etc.). And share the manager’s email signature with the PA.