You can find the serial number by pressing key combinations.
Using the built-in keyboard, press Fn + Esc.
Find the serial number in the window that opens. If a System Information window does not open, continue with these steps.
In Windows, search for and open Command Prompt.
In the command prompt window, type wmic bios get serialnumber, and then press Enter.
You can refer this HP document for more assistance:- Click here
I hope that helps.
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