How to build a "label_from" function that uses data built during ItemList preprocessing to define targets?

So I have a custom ItemList with a custom preprocessor that builds both the inputs (the X) and the targets (y) values.

I want to define a “label_from” method that simply uses those target (y) values?

Any ideas on how I need to structure this? I’ve tried a few things but nada is working so far.