For frequency feedback you would use one of the two available encoder channels to provide the tachometer signal. You can see more about connecting the encoders for feedback under the “Frequency (digital)” heading in the “Feedback Options” section of the “Pololu Jrk USB Motor Controller User’s Guide”, which is located under the “Resources” tab of the jrk 21v3 product page.
The “Module Pinout and Components” section of the same guide shows the FB pin and motor output connections on the jrk controller.
You might find this forum thread helpful in setting your jrk up to handle frequency feedback.
-Derrill