curl --request POST \
  --url https://api.example.com/api/storefront/cart/{cart_id}/discounts \
  --header 'Content-Type: application/json' \
  --header 'X-Site-ID: <x-site-id>' \
  --data '
{
  "code": "<string>"
}
'

{
  "code": "<string>",
  "kind": "<string>",
  "amount_off": "<string>",
  "currency": "<string>",
  "allocations": [
    {
      "line_item_id": "3c90c3cc-0d44-4b50-8888-8dd25736052a",
      "amount_off": "<string>"
    }
  ]
}