Cannot activate HP ROK license on Server 2012 R2 Essentials Evaluation

Hi!

Ive bought a HP ProLiant ML310e Gen8 and installed the Server 2012 R2 Essentialsevaluation, because I couldnt recieve my license in time.

Ive now recieved my Windows Server 2012 R2 Essentials ROK HP but cannot activate the license.

Due to my knowledge it should be possible. I dont want to reinstall my server :-

"(For Windows Server 2012 Essentials: You can convert to the full retail version by entering a retail, volume license, or OEM key in the command slmgr.vbs."

https://technet.microsoft.com/en-us/library/jj574204.aspx

Please help!