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 request this functionality, I recommend visiting our Product Suggestion forum.
In our Product Suggestion forum, you can submit requests and vote on suggestions other Brainshark users have made, as well as subscribe to them for updates. This lets our product team know what features users would like to see in the future.
To access the Product Suggestion forum, please sign into your Brainshark account and select "Product Suggestions" from the drop-down menu in the top right corner. This will automatically create an account for you if you don't already have one (and if you do, you will be signed in automatically).