Open Settings and then select System. Select About and check Edition. If it shows Windows Server 2019 Standard or others non-evaluation edition, you can activate it without reboot.

Can Windows Server 2019 evaluation be activated?

An evaluation version can only be activated using a retail key, if the key was from volume center then you’ll need to use volume distribution media that can be downloaded from volume licensing center.

Can you activate Windows Server 2016 evaluation?

As you know all Evaluations versions is available for testing for 180 days and after that period you must convert the Evaluation version to Licensed first and to use a valid product key in order to activate Windows Server 2016 (or Server 2019)and use all its features without problems.