How do I create a GUI Procedure in Minecraft 1.12.2?

I just can't get the GUI to work the way that I want it to. I want to have something that converts items into other items, but I think that it lacks a crucial part of the code. This is in McCreator 2020.2. The code is get item from slot 0 of the currently open GUI of Event/target entity = <item>. Is there a way to do something like this in this version?