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
}
'

require 'uri'
require 'net/http'

url = URI("https://api.kit.com/v4/bulk/forms/subscribers")

http = Net::HTTP.new(url.host, url.port)
http.use_ssl = true

request = Net::HTTP::Post.new(url)
request["Authorization"] = 'Bearer <token>'
request["Content-Type"] = 'application/json'
request.body = "{\n  \"additions\": [\n    {\n      \"form_id\": 0,\n      \"subscriber_id\": 0\n    },\n    {\n      \"form_id\": 1,\n      \"subscriber_id\": 1\n    },\n    {\n      \"form_id\": 2,\n      \"subscriber_id\": 2\n    },\n    {\n      \"form_id\": 3,\n      \"subscriber_id\": 3\n    }\n  ],\n  \"callback_url\": null\n}"

response = http.request(request)
puts response.read_body

const options = {
  method: 'POST',
  headers: {Authorization: 'Bearer <token>', 'Content-Type': 'application/json'},
  body: JSON.stringify({
    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
  })
};

fetch('https://api.kit.com/v4/bulk/forms/subscribers', options)
  .then(res => res.json())
  .then(res => console.log(res))
  .catch(err => console.error(err));

package main

import (
	"fmt"
	"strings"
	"net/http"
	"io"
)

func main() {

	url := "https://api.kit.com/v4/bulk/forms/subscribers"

	payload := strings.NewReader("{\n  \"additions\": [\n    {\n      \"form_id\": 0,\n      \"subscriber_id\": 0\n    },\n    {\n      \"form_id\": 1,\n      \"subscriber_id\": 1\n    },\n    {\n      \"form_id\": 2,\n      \"subscriber_id\": 2\n    },\n    {\n      \"form_id\": 3,\n      \"subscriber_id\": 3\n    }\n  ],\n  \"callback_url\": null\n}")

	req, _ := http.NewRequest("POST", url, payload)

	req.Header.Add("Authorization", "Bearer <token>")
	req.Header.Add("Content-Type", "application/json")

	res, _ := http.DefaultClient.Do(req)

	defer res.Body.Close()
	body, _ := io.ReadAll(res.Body)

	fmt.Println(string(body))

}

<?php

$curl = curl_init();

curl_setopt_array($curl, [
  CURLOPT_URL => "https://api.kit.com/v4/bulk/forms/subscribers",
  CURLOPT_RETURNTRANSFER => true,
  CURLOPT_ENCODING => "",
  CURLOPT_MAXREDIRS => 10,
  CURLOPT_TIMEOUT => 30,
  CURLOPT_HTTP_VERSION => CURL_HTTP_VERSION_1_1,
  CURLOPT_CUSTOMREQUEST => "POST",
  CURLOPT_POSTFIELDS => json_encode([
    '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
  ]),
  CURLOPT_HTTPHEADER => [
    "Authorization: Bearer <token>",
    "Content-Type: application/json"
  ],
]);

$response = curl_exec($curl);
$err = curl_error($curl);

curl_close($curl);

if ($err) {
  echo "cURL Error #:" . $err;
} else {
  echo $response;
}

import requests

url = "https://api.kit.com/v4/bulk/forms/subscribers"

payload = {
    "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": None
}
headers = {
    "Authorization": "Bearer <token>",
    "Content-Type": "application/json"
}

response = requests.post(url, json=payload, headers=headers)

print(response.text)

{
  "subscribers": [
    {
      "id": 640,
      "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": 640,
      "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": 215,
        "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": 640,
        "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": [
    "The access token is invalid"
  ]
}

{
  "errors": [
    "This request exceeds your queued bulk requests limit. Please wait while we process your existing requests and try again later."
  ]
}

{
  "errors": [
    "No additions included for processing"
  ]
}