An itemUseFunction is called whenever the associated item is used on a block.

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.IItemUse;

The IItemUse function is a function with the following parameters (In this order):

• ICTPlayer player → The player doing the right-click
• IWorld world → The world the player is in
• IBlockPos pos → The Position of the block the item is used on
• Hand hand → The used Hand (main or off)
• Facing facing → The side of the block the item is used on
• Position3f blockHit → The block's relative X,Y and Z coordinate → All three are between 0 and 1

The function needs to return an ActionResult object.

ZenScript
 Copy
#loader contenttweaker

import mods.contenttweaker.VanillaFactory;
import mods.contenttweaker.ActionResult;

var item = VanillaFactory.createItem("fake_flint");

item.maxStackSize = 1;
item.maxDamage = 50;
item.onItemUse = function(player, world, pos, hand, facing, blockHit) {
    var firePos = pos.getOffset(facing, 1);
    if (world.getBlockState(firePos).isReplaceable(world, firePos)) {
        world.setBlockState(<block:minecraft:fire>, firePos);
        player.getHeldItem(hand).damage(1, player);
        return ActionResult.success();
    }

    return ActionResult.pass();
};
item.register();