In the US, IEC320 C14 plug connections are common, but use your preferred standard.
I've also seen where this was attached in this thingiverse project.
Inside the switch I've also added an F4AL250V fuse.
Now what's confusing me is, that the this rocker switch on the back reads 10A 250V~ so I'm wondering if it's safe to use.
The switch will be connected to the house mains via a plug and wired just like in the this thingiverse project, so it can be the input into my OEM Power Supply via the black (hot - L), white (Common - N) and Green (Ground) wires.
For the connecting wires I stripped a black PC Tower wire to get at the black, white, and ground wires inside it.
At 250V a rating for 10A means 2500 Watts, which is fairly enough to use with a typical 3D printer. Make sure that the wire you use to connect the switch to the power supply can also take the current you need. Easiest way to get a cable that is safe to use is salvaging a mains cable from other devices. I would not use wires that were made for low power use and with unsolid connections.
*edit: I'm still not sure which cable you intended to use from this tower PC. If it is a mains cable like this
you should be on the safe side.