The way these usually work is that without the potentiometer connected, there is an internal impedance circuit that sets a default set point, and these have quite high value, such that when one connects an external potentiometer of much lower impedance, it will take the higher current, and assert itself to dominate.
The manual should indicate suitable potentiometer values, and/or what current the electronics can supply. As example, suppose the reference voltage across the whole potentiometer is 10V, and the circuit could supply (say) 20mA.
If you chose (say) a 5K potentiometer, it would use 10V/5000ohms = 0.002A which is 2mA. That is then well within the reference supply capability.
A choice of anywhere between 5K and about 20K will normally work just fine. Do be sure that the potentiometer is linear type. Those made for audio volume control applications have a logarithmic law that would drive one nuts! You want the speed to be proportional to the knob rotation.