To convert your windows server 2019 from Evaluation to Retail, use the following PowerShell command:
DISM /Online /Set-Edition:ServerStandard /ProductKey:N69G4-B89J2-4G8F4-WWYCC-J464C /AcceptEula
For no restart prompt after the conversation, please use the following command:
DISM /NoRestart /Online /Set-Edition:ServerStandard /ProductKey:N69G4-B89J2-4G8F4-WWYCC-J464C /AcceptEula