Many newer PCs this days come with preinstalled Windows 8 or higher, and have the Windows activation key burned into the BIOS.
This allows to reinstall the same version of Windows – usually Core edition – without having to enter any activation keys. This key stays intact even when you update or reflash BIOS firmware, but you may want to have it written down in a safe place, just in case. Windows does not allow to do that, so we will use a third party software. I recommend Aida64 which is a paid piece of software, but it comes with a 30-day trial period which is more than enough for our task. Run the utility and go to Motherboard, then ACPI. Here, scroll down to MSDM and find the line called SLS Data. The value of this line is the activation key of shipped preinstalled copy of Windows. Copy and paste it to a file and save somewhere safe. Aida64 seems to be the easiest utility to extract Windows activation key from Windows, but there are other utilities as well, like this free one.