IItemStackSupplier
Link to iitemstacksupplier
The IItemStackSupplier function can be added to a Creative Tab and is used to provide the IItemStack to be used as the tab's icon.
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.IItemStackSupplier;
Parameters
Link to parameters
The IItemStackSupplier Function is a function with no parameters. The function needs to return an IItemStack.