Processes in Microsoft 365 for setting up Office apps, redeeming product keys, and activating licenses.
I assume you purchased it from a third-party reseller, correct?
If you've lost your product key and cannot retrieve it, but still have the receipt, I recommend contacting the retailer immediately and explaining what happened. They usually keep records of the products they sell. If they cannot retrieve the key themselves, they may have a direct way to contact Microsoft Sales and request assistance in recovering it.