Link to iresourcelocationsupplier

The IResourceLocationSupplier function can be used to provide the Resource Location to be used for whatever.

Importing the package

Link to importing-the-package

It might be required for you to import the package if you encounter any issues, so better be safe than sorry and add the import.
import mods.contenttweaker.IResourceLocationSupplier;

The IResourceLocationSupplier Function is a function with no parameters.
The function needs to return an Resource Location.