curl --request GET \
  --url https://api.kit.com/v4/custom_fields \
  --header 'X-Kit-Api-Key: <api-key>'

require 'uri'
require 'net/http'

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

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

request = Net::HTTP::Get.new(url)
request["X-Kit-Api-Key"] = '<api-key>'

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

const options = {method: 'GET', headers: {'X-Kit-Api-Key': '<api-key>'}};

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

package main

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

func main() {

	url := "https://api.kit.com/v4/custom_fields"

	req, _ := http.NewRequest("GET", url, nil)

	req.Header.Add("X-Kit-Api-Key", "<api-key>")

	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/custom_fields",
  CURLOPT_RETURNTRANSFER => true,
  CURLOPT_ENCODING => "",
  CURLOPT_MAXREDIRS => 10,
  CURLOPT_TIMEOUT => 30,
  CURLOPT_HTTP_VERSION => CURL_HTTP_VERSION_1_1,
  CURLOPT_CUSTOMREQUEST => "GET",
  CURLOPT_HTTPHEADER => [
    "X-Kit-Api-Key: <api-key>"
  ],
]);

$response = curl_exec($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/custom_fields"

headers = {"X-Kit-Api-Key": "<api-key>"}

response = requests.get(url, headers=headers)

print(response.text)

{
  "custom_fields": [
    {
      "id": 1,
      "name": "ck_field_1_last_name",
      "key": "last_name",
      "label": "Last name"
    }
  ],
  "pagination": {
    "has_previous_page": false,
    "has_next_page": false,
    "start_cursor": "WzFd",
    "end_cursor": "WzFd",
    "per_page": 500
  }
}

{
  "errors": [
    "The access token is invalid"
  ]
}