If your notebook came with W10 from HP, the product key is in the BIOS.
You don't need it or even need to know what it is to reinstall W10.
However, if you want to see the product key and the version of W10 the key is good for, you can install the free Showkey Plus software from the Microsoft Store and it will generate a report for you that you can print out and save or save as a text file.
Be alert for scammers posting fake support phone numbers and/or email addresses on the community. If you think you have received a fake HP Support message, please report it to us by clicking on "Flag Post".