That depends - if the integration means opening a SDM page in a frame/iframe, then it cannot be 100% 'hidden', because the user's browser will have to connect to the SDM page. If you're using an integration based on WebServices to SDM, then the URL would not appear anywhere since the portal will take care of communicating with SDM (server-side).
Why would you want the URL to be 'hidden' ? How are you actually doing the integration ?