Shopify Javascript - Add item to cart for free -Shopify Script

discounted_product = 12275195905
products_needed = [592406273]
products_seen = []

Input.cart.line_items.each do |line_item|
  product = line_item.variant.product
  products_seen << product.id if products_needed.include?(product.id)
end

Input.cart.line_items.each do |line_item|
  product = line_item.variant.product
  next unless product.id == discounted_product
  line_item.change_line_price(line_item.line_price * 0.0, message: "Free Product with Purchase") if products_seen.uniq.sort == products_needed.uniq.sort
end

Output.cart = Input.cart

Hi how can I use this?

I have a shopify store and want to automatically add a free gift to the customers cart if customer has more than 30$ in their cart.
Where do I need to place this ?