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

require 'uri'
require 'net/http'

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

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  \"custom_fields\": [\n    {\n      \"label\": \"Test Custom Field 0\"\n    },\n    {\n      \"label\": \"Test Custom Field 1\"\n    },\n    {\n      \"label\": \"Test Custom Field 2\"\n    },\n    {\n      \"label\": \"Test Custom Field 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({
    custom_fields: [
      {label: 'Test Custom Field 0'},
      {label: 'Test Custom Field 1'},
      {label: 'Test Custom Field 2'},
      {label: 'Test Custom Field 3'}
    ],
    callback_url: null
  })
};

fetch('https://api.kit.com/v4/bulk/custom_fields', 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/custom_fields"

	payload := strings.NewReader("{\n  \"custom_fields\": [\n    {\n      \"label\": \"Test Custom Field 0\"\n    },\n    {\n      \"label\": \"Test Custom Field 1\"\n    },\n    {\n      \"label\": \"Test Custom Field 2\"\n    },\n    {\n      \"label\": \"Test Custom Field 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/custom_fields",
  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([
    'custom_fields' => [
        [
                'label' => 'Test Custom Field 0'
        ],
        [
                'label' => 'Test Custom Field 1'
        ],
        [
                'label' => 'Test Custom Field 2'
        ],
        [
                'label' => 'Test Custom Field 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/custom_fields"

payload = {
    "custom_fields": [{ "label": "Test Custom Field 0" }, { "label": "Test Custom Field 1" }, { "label": "Test Custom Field 2" }, { "label": "Test Custom Field 3" }],
    "callback_url": None
}
headers = {
    "Authorization": "Bearer <token>",
    "Content-Type": "application/json"
}

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

print(response.text)

{
  "custom_fields": [
    {
      "id": 144,
      "key": "existing_custom_field",
      "label": "Existing Custom Field",
      "name": "ck_field_144_existing_custom_field",
      "created_at": "2023-02-17T11:43:55Z"
    },
    {
      "id": 146,
      "key": "interests",
      "label": "Interests",
      "name": "ck_field_146_interests",
      "created_at": "2023-02-17T11:43:55Z"
    },
    {
      "id": 145,
      "key": "last_name",
      "label": "Last name",
      "name": "ck_field_145_last_name",
      "created_at": "2023-02-17T11:43:55Z"
    }
  ],
  "failures": []
}

{}

{
  "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 custom fields included for processing"
  ]
}