Had to use the Quick Restore and afterwards when starting Windows98 asked for Product Key. Don't have since it was factory installed, had no disk except Compaq Quick Restore
The "Product Key" should be on a sticker somewhere on the case. Dell Optiplex 780/intel Core2 Quad 3.0ghz/ 16gb Crucial DDR3L-1600/MSI GT-710 1gb PCIe/SB Audigy FX 5.1 PCIe/Cooler Master RS500/Win10 Pro 64bit
You could also try the free utility linked below. The latest version also can show installation keys for other products if they are recorded in the Windows registry.