I can't seem to figure out how to name the data to put it into RollingWindow. I am sure, the solution should be intuitive, but I am not having that moment of clarity now.

Basically I am just trying to find the volume of prior bar. That simple. I started using RollingWindow, based on example in QCU and on examples in Forum. But I can't seem to add data to the RollingWindow, no matter what I try. I also tried Add(data ["ES"]) , but so far nothing works. I would appreciate your advise.