It's called an SMA connector and you'd have to solder it to the point at which the internal antenna used to be (which you'd have to remove unless you want to do some very complex calculations to run both the internal and external antennas in phase and at the correct terminal impedance).
If you haven't done microwave work before, I don't suggest even thinking about it. Even a grainy solder joint at those frequencies can introduce standing waves that, aside from making the signal worse, will burn out the transmitter. (You're dealing with quarter wavelengths of around 3cm. A bad solder connection 3cm from the transmitter is a short.)