BioS embedded product Key needs to be viewable

To update Windows 8.1 I need the windows Product key which is not packaged with my Dell Inspiron All In One. Is great it is encoded in BIOS but where can I get a written copy so I can answer product key question in control panel. Even though I have a legit MSDN product key for Win 8.1 it refuses it so I need the original. Is kind of aggravating that there is not an obvious way to view it.