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

{
  "subscribers": [
    {
      "id": 616,
      "first_name": "Sub",
      "email_address": "sub@example.com",
      "created_at": "2023-02-17T11:43:55Z",
      "added_at": "2023-02-17T11:43:55Z",
      "referrer": "https://mywebsite.com/bfpromo/?utm_source=facebook&utm_medium=cpc&utm_campaign=black_friday&utm_term=car_owners&utm_content=get_10_off",
      "referrer_utm_parameters": {
        "source": "facebook",
        "medium": "cpc",
        "campaign": "black_friday",
        "term": "car_owners",
        "content": "get_10_off"
      }
    },
    {
      "id": 616,
      "first_name": "Sub",
      "email_address": "sub@example.com",
      "created_at": "2023-02-17T11:43:55Z",
      "added_at": "2023-02-17T11:43:55Z",
      "referrer": "https://mywebsite.com/bfpromo/?utm_source=facebook&utm_medium=cpc&utm_campaign=black_friday&utm_term=car_owners&utm_content=get_10_off",
      "referrer_utm_parameters": {
        "source": "facebook",
        "medium": "cpc",
        "campaign": "black_friday",
        "term": "car_owners",
        "content": "get_10_off"
      }
    }
  ],
  "failures": [
    {
      "errors": [
        "Subscriber does not exist"
      ],
      "subscription": {
        "form_id": 124,
        "subscriber_id": null,
        "referrer": "https://mywebsite.com/bfpromo/?utm_source=facebook&utm_medium=cpc&utm_campaign=black_friday&utm_term=car_owners&utm_content=get_10_off"
      }
    },
    {
      "errors": [
        "Form does not exist"
      ],
      "subscription": {
        "form_id": null,
        "subscriber_id": 616,
        "referrer": "https://mywebsite.com/bfpromo/?utm_source=facebook&utm_medium=cpc&utm_campaign=black_friday&utm_term=car_owners&utm_content=get_10_off"
      }
    }
  ]
}