How to Compare Power Data? Kickr Core vs Assioma's

I’m close but not quite. Getting the custom chart was easy. Ends up just being a drop down for my new field: Power2.

Now I’m trying to create a Custom Field that shows up in the interval summary. This requires some scripting but I don’t know how to do it. I’ve done some coding so I know the general approach but any time I use this post (https://forum.intervals.icu/t/server-side-data-model-for-scripts/) to help me figure out which keyword to use I get a "ReferenceError: “xxx” is not defined.

I’ve tried the following
CustomItem.Power2
Interval.Power2
ActivityStream.Power2

I’m obviously not understanding how to use custom stream fields. Could someone dumb this down for me?
BTW, it’s easy to get the data I’m looking for (Power meter 1 avg interval vs power meter 2 avg interval) elsewhere in intervals.icu, but I’d like to figure out this coding aspect.