Get Feedback from Internal Potentiomet of Servo Motor
Generally speaking questions about modification of undocumented devices are not allowed here.

That said, it is likely that the servo amplifier wires the variable resistor as a potentiometer, which is to say applies some relatively constant voltage to the outer leads and measures the voltage on the inner lead.

If you want to "snoop" on the position you would need to connect a high-impedance ADC input to the middle lead, and perhaps also the the higher voltage outer lead to measure the scale.

Do take note that some MCU ADC inputs are not high impedance at all - especially in faster conversion modes, they can draw a fair amount of burst current.  If that is the case you may need a buffer amplifier to avoid disturbing the servo amplifier's own reading.

There are also various retrofit designs/kits out there which replace the electronics of common servos with an MCU based solution, often offering a more digital communication scheme. You could potentially use one of these and modify it to report position back over a bidirectional interface scheme.