This has been answered here on our LibAnswers page (answer is copied below for your convenience)
First, be certain you are using the “off campus” Films on Demand: Master Academic Collection link. You can recognize proxied or off campus links by the 0- at the front of the url, and the .library.acaweb.org after the domain (ex: http://0-digital.films.com.library.acaweb.org/Dashboard.aspx). Being certain you are using the off campus url ensures that you find the right link that will work on or off campus.
Once you are in Films on Demand, the easiest way to find the Persistent URL is to use the citation option under the video:
Which when you click on it will give you the PURL as part of the citation: