Are you wanting the Windows 8 key or Windows 10 key? If you did the free upgrade, that PC now has a digital entitlement. You just install that same version of Windows 10 and click skip when asked for a key. It will activate with the digital entitlement. The key that goes with the digital entitlement is a generic key. There is one for Pro, one for Home, one for SL etc. You don't really need it.
If that PC shipped from the factory with Windows 8 or 8.1 on it, that key is embedded in the BIOS. Windows 8.1 install media will read it and use it automatically, assuming you use the correct install media. showkey should show you your embedded key and what version of Windows 8 or 8.1 its for.
Computer Type PC/Desktop System Manufacturer/Model Number Pi Foundation Raspberry Pi 3B OS Windows 10 IoT CPU ARM Cortex-A53 1.2 GHz 64 bit quad-core Motherboard Raspberry Pi Model 3B Memory 1 GB (LPDDR2 SDRAM) (shared with GPU) Graphics Card •VideoCore IV multimedia/3D graphics core @ 400MHz/300MHz Sound Card Combined 3.5mm audio jack and composite video Monitor(s) Displays 7 Inch Pi Foundation Touch Screen Screen Resolution 800x480 @60fps 24-bit color,
Keyboard Mini Wireless with touchpad Mouse FT5406 10 point capacitive touchscreen PSU 5V @ 10 A Case Pibow Midnight - Limited-Edition! Cooling 7.5 mm Heat Sink no fan. Hard Drives 8 GB SanDisk Ultra Class 10 Micro SD card. Internet Speed 10/100 Mbit/s Ethernet - 802.11n wireless - Bluetooth 4.1 Browser non Antivirus non Other Info •4 USB ports
•40 GPIO pins
•Full size HDMI 1.3a port
•Camera interface (CSI)
•Display interface (DSI)
•Micro SD card slot
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