Using iMac late 2013 as external monitor to HP EliteBook 820 G3 on Windows 10
This is not possible.

In order to use your iMac as a display (Target Display Mode) , you need to connect it via a Thunderbolt 1 or 2 port.  The HP laptop you listed does not have a Thunderbolt port per the technical specifications.

Software Based Solutions


AirParrot 2. This allows you to "beam" your screen to any supported device
SpaceDesk.  This software allows you to extend your Windows desktop to another computer or browser over the network.