Hi Adolf Daud,
To put it simply: you don't need one right now. Since you are setting this up in VirtualBox (likely for testing, learning, or a lab environment), you can bypass that prompt entirely.
When the setup wizard asks you to enter the product key, just look at the bottom of that window and click the "I don't have a product key" link.
This will allow you to skip the screen and proceed with the installation. Windows Server will install in a 180-day Evaluation mode, which gives you full access to all features for testing. If you ever decide to use it permanently later, you can simply enter a purchased key within the Windows Settings.
I hope this helps you get your VM running! If this answers your question, please click "Accept Answer".
Tracy.