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;

The IItemStackSupplier Function is a function with no parameters. The function needs to return an IItemStack.