variable control with inverter

I use a VFD to run a hi frequency spindle. I have always set my
speed[freq.] via the digital keypad.
However, the inverter will accept analog signals for speed control,
either 8 pre-sets or proportional for true variable.
What do I need to buy/build to use something like a 10 turn pot
so I can adjust on the "fly"?

thanks
Carl