The documentation here says that the correct property to define a scope for the relation field is ‘modelScope’, but that doesn’t work, you need to use ‘scope’
I don’t know if this is a bug in the docs or in the code.
This is the relevant code in the relation widget, you can see that it only expects the ‘scope’ property and won’t do anything with the ‘modelScope’ property:
I’m using October v3.7.12