I have two lists-
Master and the Child list
I am using the Master List's ID column and I created an integer column in the Child List called "CaseID". CaseID is my Key Column.
Here is the Flow.
ChildList-Column eq 'MasterList Column'
In my case:
Your Master List Column should be able to be added from the Dynamic Content from the "When an item is created or modified" section.
"Value" in the Apply to each comes from your "Get Items" section in the Dynamic Content.
The Values for your "Update Item" should come from the "When an item is created or modified" section in the Dynamic Content.
The Values for your "Create Item" should come from the "When an item is created or modified" section in the Dynamic Content.