A product key is a unique 5-digit code used during installation to distinguish between a standalone software purchase and one that is part of a larger product suite. For AutoCAD 2013, the standard standalone product key is . Common AutoCAD 2013 Product Keys
If you have already installed the software as a trial and need to activate it later, you can update these details in the License Manager window within the application, as noted in the Autodesk Support documentation Why the Product Key Matters product key of autocad 2013
