Correct license (key) to purchase to change from 2012 R2 x64 Trial to production?

I setup a Trial version of Server 2012 R2 x 64 Essentials. The project was placed on hold. The trial expired. The project was restarted and I need to convert the trial license to a production license. I have production 2012 R2 keys, however none will activate this install.  What is the correct P/N or reference to purchase the key to convert the system from trail to production?

Mike