This would basically add an extra arguments to allow adding a glow effect to an item you make, or make the item variant of a block glow as well.

Item version:
"var name".itemGlowing = true/false;

Example:

import mods.contenttweaker.VanillaFactory;
import mods.contenttweaker.Item;

var zsItem = VanillaFactory.createItem("zs_item");
zsItem.maxStackSize = 1;
zsItem.rarity = "rare";
zsItem.beaconPayment = true;
zsItem.itemGlowing = true;
zsItem.register();

Read the docs. Glowing is already a property

What is the exact method for it then.

glowing