curl --request POST \
  --url https://api.kit.com/v4/forms/{form_id}/subscribers \
  --header 'Content-Type: application/json' \
  --header 'X-Kit-Api-Key: <api-key>' \
  --data '
{
  "email_address": "alice@convertkit.dev",
  "referrer": "https://mywebsite.com/bfpromo/?utm_source=facebook&utm_medium=cpc&utm_campaign=black_friday&utm_term=car_owners&utm_content=get_10_off"
}
'

require 'uri'
require 'net/http'

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

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

request = Net::HTTP::Post.new(url)
request["X-Kit-Api-Key"] = '<api-key>'
request["Content-Type"] = 'application/json'
request.body = "{\n  \"email_address\": \"alice@convertkit.dev\",\n  \"referrer\": \"https://mywebsite.com/bfpromo/?utm_source=facebook&utm_medium=cpc&utm_campaign=black_friday&utm_term=car_owners&utm_content=get_10_off\"\n}"

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

const options = {
  method: 'POST',
  headers: {'X-Kit-Api-Key': '<api-key>', 'Content-Type': 'application/json'},
  body: JSON.stringify({
    email_address: 'alice@convertkit.dev',
    referrer: 'https://mywebsite.com/bfpromo/?utm_source=facebook&utm_medium=cpc&utm_campaign=black_friday&utm_term=car_owners&utm_content=get_10_off'
  })
};

fetch('https://api.kit.com/v4/forms/{form_id}/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/forms/{form_id}/subscribers"

	payload := strings.NewReader("{\n  \"email_address\": \"alice@convertkit.dev\",\n  \"referrer\": \"https://mywebsite.com/bfpromo/?utm_source=facebook&utm_medium=cpc&utm_campaign=black_friday&utm_term=car_owners&utm_content=get_10_off\"\n}")

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

	req.Header.Add("X-Kit-Api-Key", "<api-key>")
	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/forms/{form_id}/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([
    'email_address' => 'alice@convertkit.dev',
    'referrer' => 'https://mywebsite.com/bfpromo/?utm_source=facebook&utm_medium=cpc&utm_campaign=black_friday&utm_term=car_owners&utm_content=get_10_off'
  ]),
  CURLOPT_HTTPHEADER => [
    "Content-Type: application/json",
    "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/forms/{form_id}/subscribers"

payload = {
    "email_address": "alice@convertkit.dev",
    "referrer": "https://mywebsite.com/bfpromo/?utm_source=facebook&utm_medium=cpc&utm_campaign=black_friday&utm_term=car_owners&utm_content=get_10_off"
}
headers = {
    "X-Kit-Api-Key": "<api-key>",
    "Content-Type": "application/json"
}

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

print(response.text)

{
  "subscriber": {
    "id": 788,
    "first_name": "Alice",
    "email_address": "alice@convertkit.dev",
    "state": "active",
    "created_at": "2023-02-17T11:43:55Z",
    "added_at": "2023-02-17T11:43:55Z",
    "fields": {},
    "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"
    }
  }
}

{
  "subscriber": {
    "id": 789,
    "first_name": "Alice",
    "email_address": "alice@convertkit.dev",
    "state": "active",
    "created_at": "2023-02-17T11:43:55Z",
    "added_at": "2023-02-17T11:43:55Z",
    "fields": {},
    "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"
    }
  }
}

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

{
  "errors": [
    "Not Found"
  ]
}

{
  "errors": [
    "Either subscriber id or email address is required to add subscriber to form"
  ]
}