Get Checkout ID/Token from Carrier Service

[KHCoolrunner]

Hi Shopify Community,

Im working with an app that will provide shipping rates for Shopify shops, but i ran into a problem i hope you can help me with.

The problem is that, we define shipping rules on our platform regarding to the Shopify order data from the Carrier Service, but i need to know if a discount was added or some promocode, but i can't see that in the Carrier Service request. Below is an example of this request:

{
  "rate": {
    "origin": {
      "country": "DK",
      "postal_code": "9400",
      "province": null,
      "city": "N\u00f8rresundby",
      "name": null,
      "address1": "Stigsborgvej 60",
      "address2": null,
      "address3": null,
      "phone": null,
      "fax": null,
      "email": null,
      "address_type": null,
      "company_name": "coolsc"
    },
    "destination": {
      "country": "DK",
      "postal_code": "9400",
      "province": null,
      "city": "N\u00f8rresundby",
      "name": "Kevin Test",
      "address1": "Testvej 31",
      "address2": "ST 3",
      "address3": null,
      "phone": null,
      "fax": null,
      "email": null,
      "address_type": null,
      "company_name": null
    },
    "items": [
      {
        "name": "Test produkt",
        "sku": "test100",
        "quantity": 1,
        "grams": 1200,
        "price": 10000,
        "vendor": "CoolSC",
        "requires_shipping": true,
        "taxable": true,
        "fulfillment_service": "manual",
        "properties": null,
        "product_id": 5714741231771,
        "variant_id": 36401958027419
      }
    ],
    "currency": "DKK",
    "locale": "da"
  }
}

 As you can see im not able to see that an discount was added in this checkout. I noticed i have to get the Checkout object to know this, but my question is "How do i know the Checkout ID/Token, so i can find the checkout?"

Hope you guys can help me out here.