This post contains steps to install product key and upgrade Microsoft System Center 2012 Configuration Manager from evaluation version to fully license version.
Follow the below steps to install product key on ConfigMgr 2012:
Start Configuration Manager Setup from start menu:
When Microsoft System Center 2012 Configuration Manager Setup Wizard start Select Perform site maintenance or reset Site option and click next
Select Convert from Evaluation to Full Product Version. Enter your 25-character product key, write product key and click next.
Accept the license agreement and click next.
Wait until it finish and make sure that Core setup has completed is appear
and click close