The FET wire in the KO servo is for connecting to 7.2V +. So, you should connect it to the + size of you battery (or the plug). OR, If your receiver doesn't have BEC, you can connect the FET wire to the + of your reveiver.
If your receiver have BEC and you connect it to the + of your receiver, +5V will be provied to the servo. It still works but the speed is low.