Automatic power off circuit for battery charger
Hystersis is positive feedback. You  may know you can get negative feedback by connecting the amplifier output with a resistor to the inverting input. So you can get positive feedback by connecting a resistor to the positive input. Sometimes a small capacitor across the feedback resistor, perhaps 100 pF or so, is useful to speed up the transition.

If the signal coming into the +ve input was from a voltage source, then this wouldn't work. However, the input is from an effective resistance of a little below 1 k&Omega; to a couple of k&Omega; depending on the position of the potentiometer. That variability will make the size of the hysteresis window dependent on the pot position.

Andy's suggestion of 100 k&Omega; is a good place to start for the feedback resistor, and will give you a hysteresis of one percent or two of the opamp output swing.