curl --request POST \
  --url https://api.kit.com/v4/bulk/subscribers \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '{
  "subscribers": [
    {
      "first_name": "Test Subscriber 0",
      "email_address": "[email protected]"
    },
    {
      "first_name": "Test Subscriber 1",
      "email_address": "[email protected]"
    },
    {
      "first_name": "Test Subscriber 2",
      "email_address": "[email protected]"
    },
    {
      "first_name": "Test Subscriber 3",
      "email_address": "[email protected]"
    }
  ],
  "callback_url": null
}'

{
  "subscribers": [
    {
      "id": 15964,
      "first_name": null,
      "email_address": "[email protected]",
      "state": "active",
      "created_at": "2023-02-17T11:43:55Z"
    },
    {
      "id": 15965,
      "first_name": "Camille",
      "email_address": "[email protected]",
      "state": "active",
      "created_at": "2023-02-17T11:43:55Z"
    },
    {
      "id": 15963,
      "first_name": "Alice",
      "email_address": "[email protected]",
      "state": "active",
      "created_at": "2023-02-17T11:43:55Z"
    }
  ],
  "failures": [
    {
      "subscriber": {
        "first_name": "Benito",
        "email_address": null,
        "state": "active",
        "created_at": null
      },
      "errors": [
        "Email address is invalid"
      ]
    }
  ]
}