For some special processes we downloading the evaluation version of Windows Server and want to use permanently with existing settings. In this case we need to enter a valid license for conversation.
But there is a key point in here. First of all you need to convert the Evaluation to Standart or Datacenter edition. For this process open Command Prompt and enter that command line :
Dism /online /Set-Edition:ServerStandard /AcceptEula /ProductKey:12345-67890-12345-67890-12345
I mentioned the product key as 12345 but you need to type your own valid key in here.
This process will be take a time. You need to be patient in this process. If you want convert to Datacenter Edition you need to replace ServerStandard to ServerDatacenter.