How come it doesn't has a power supply? by reading the specs it should have a 450W PSU:
https://www3.lenovo.com/us/en/desktops-and-all-in-ones/ideacentre/y700/IdeaCentre-Y710-Cube/p/99IC9Y10247
The auxiliary 6 pin power connector should be there, open the other side of the case...