If you have a piece of Brainshark content that was created only by uploading multimedia files (such as an MP4 file) it is not possible at this time to add URL attachments to the presentation.
Possible workarounds include adding the URL link as a URL slide with the "Add A Slide" functionality. You can also enable the Completion Criteria and enable Completion Branching to send the user to the URL link once the completion criteria has been met.
If you would like to see this functionality implemented in a future release, please reach out to your Customer Success Manager.