To activate your Windows Server 2019, take the following steps:
Using Server Manager
Step 1 : Open Server Manager from the Start menu. And select Local Server.
Step 2 : In the Properties section, click the highlighted Not activated setting of the product ID.
Step 3 : In the Enter a product key window, enter your Windows Server 2019 product key and press Enter.
Step 4 : Microsoft’s Activation Server checks the product key you entered. If it is valid, click Next in the Activate Windows window.
Step 5 : When activation finishes, click Close to close the Thank you for activating window.
Using Server Configuration
Step 1 : Type Sconfig.cmd, and then press ENTER. The Server Configuration tool interface opens.
Step 2 : Enter 11 as a selected option and press Enter.
Step 3 : Enter 3 in the sub-menu to install the product key and press Enter.
Step 4 : In the Enter Product Key window, enter the Windows Server 2019 product key and click OK.
Step 5 : Enter 2 from the sub-menu to activate Windows and press Enter.
Step 6 : After a short time, your Windows Server 2019 will be activated. Enter Exit to close the activation window.
Step 7 : Enter 4 from the sub-menu to exit and return to the main menu.