There is currently no way to determine the URL in advance of the file's processing.
Our mixing server can be configured (in the Recording Configuration tab for your App Key on your Temasys Console) to send a POST request to your API server, which you can then use to forward information to your users. The download link for the file is sent to your API server when the recording is ready.
Here's a guide you may wish to refer to: http://support.temasys.com.sg/a/solutions/articles/12000036047-recording-notification-api-call