Safely Hooking a Switch up to an OEM Power Supply?
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 
https://www.google.com/search?q=mains+power+cable&amp;source=lnms&amp;tbm=isch&amp;sa=X&amp;ved=0ahUKEwjL6Y23l9zLAhUCrRoKHXvsArsQ_AUIBygB&amp;biw=1421&amp;bih=922
you should be on the safe side.