Most speed controls have an extra wire coming from the case, that wire is the 7.2 volt tap for the 7.2 volt FET servos like the KO 2123.
The 2123 will also have an extra wire coming from its case. All you'll need to do is connect the extra wire from the servo to the extra wire from the speed control and you've got your 7.2 volt feeding the FETS in the servo. There will be a small choke (looks like a resistor) that goes between the two wires where they connect, it comes packaged with the servo.
If you have a speed control that doesn't have the extra wire, you can tap off the positive wire from the battery to the speed control, that's also 7.2 volts (assuming you run a 6 cell battery).
For what its worth, I use the KO 2143, almost identical to the 2123 but runs at 6 volts and doesn't require the extra wiring. I've used both, and I cna't say the difference was worth the extra hassle.