Hi Paul,
the proper connector for the FM aerial socket is a female IEC aerial plug. Any decent electronics store should carry them. Failing that, get a TV antenna cord and snap off the wrong (male plug) end. Then strip ~75 cm of the free end, separate the center conductor and the sleeve, spread them out and you have a quite competitive FM antenna.
But, it's far from critical and it doesn't matter if you happen to short the socket. Any length of wire you manage to wrap around the center connector in the socket will probably work for strong and medium stations.