You're looking for a product key for AutoCAD 2013. I must emphasize that sharing or using unauthorized product keys is against the terms of service of Autodesk, the software company behind AutoCAD, and may violate copyright laws.
While product keys are universal for the version, serial numbers are unique to your license. They generally follow a three-digit prefix followed by an eight-digit number (e.g., 367-XXXXXXXX ). You can find this in your portal or on the physical packaging if you have a legacy disk. Product Keys for 2013 Design Suites product key of autocad 2013
If you are installing a suite (e.g., Autodesk Building Design Suite 2013), each component inside the suite has its own key, but the suite itself uses a master product key (e.g., 448E1 for the Building Design Suite Premium 2013). You're looking for a product key for AutoCAD 2013
Check the physical folder or the back of the DVD case if you have a boxed version. Look for a text file (often named Launch AutoCAD 2013 : Start AutoCAD 2013 on your computer