const response = await fetch('https://api.emailit.com/v2/audiences/aud_1234567890/subscribers', {
  method: 'POST',
  headers: {
    'Authorization': 'Bearer em_test_51RxCWJ...vS00p61e0qRE',
    'Content-Type': 'application/json'
  },
  body: JSON.stringify({
    email: 'john.doe@example.com',
    first_name: 'Іван',
    last_name: 'Петренко',
    custom_fields: {
      company: 'ТОВ "Прогрес"',
      role: 'Розробник'
    }
  })
});

const result = await response.json();

<?php

$curl = curl_init();

curl_setopt_array($curl, [
  CURLOPT_URL => 'https://api.emailit.com/v2/audiences/aud_1234567890/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' => 'john.doe@example.com',
    'first_name' => 'Іван',
    'last_name' => 'Петренко',
    'custom_fields' => [
      'company' => 'ТОВ "Прогрес"',
      'role' => 'Розробник'
    ]
  ]),
  CURLOPT_HTTPHEADER => [
    'Authorization: Bearer em_test_51RxCWJ...vS00p61e0qRE',
    'Content-Type: application/json'
  ],
]);

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

curl_close($curl);

if ($err) {
  echo "Помилка cURL #:" . $err;
} else {
  $result = json_decode($response, true);
  print_r($result);
}

import requests

response = requests.post(
    'https://api.emailit.com/v2/audiences/aud_1234567890/subscribers',
    json={
        'email': 'john.doe@example.com',
        'first_name': 'Іван',
        'last_name': 'Петренко',
        'custom_fields': {
            'company': 'ТОВ "Прогрес"',
            'role': 'Розробник'
        }
    },
    headers={
        'Authorization': 'Bearer em_test_51RxCWJ...vS00p61e0qRE',
        'Content-Type': 'application/json'
    }
)

result = response.json()
print(result)

require 'net/http'
require 'json'

uri = URI('https://api.emailit.com/v2/audiences/aud_1234567890/subscribers')
http = Net::HTTP.new(uri.host, uri.port)
http.use_ssl = true

request = Net::HTTP::Post.new(uri)
request['Authorization'] = 'Bearer em_test_51RxCWJ...vS00p61e0qRE'
request['Content-Type'] = 'application/json'
request.body = {
  email: 'john.doe@example.com',
  first_name: 'Іван',
  last_name: 'Петренко',
  custom_fields: {
    company: 'ТОВ "Прогрес"',
    role: 'Розробник'
  }
}.to_json

response = http.request(request)
result = JSON.parse(response.body)
puts result

package main

import (
    "bytes"
    "encoding/json"
    "fmt"
    "net/http"
)

func main() {
    url := "https://api.emailit.com/v2/audiences/aud_1234567890/subscribers"
    
    data := map[string]interface{}{
        "email": "john.doe@example.com",
        "first_name": "Іван",
        "last_name": "Петренко",
        "custom_fields": map[string]string{
            "company": "ТОВ \"Прогрес\"",
            "role": "Розробник",
        },
    }
    
    jsonData, _ := json.Marshal(data)

    req, _ := http.NewRequest("POST", url, bytes.NewBuffer(jsonData))
    req.Header.Set("Authorization", "Bearer em_test_51RxCWJ...vS00p61e0qRE")
    req.Header.Set("Content-Type", "application/json")

    client := &http.Client{}
    resp, _ := client.Do(req)
    defer resp.Body.Close()

    var result map[string]interface{}
    json.NewDecoder(resp.Body).Decode(&result)
    fmt.Println(result)
}

use reqwest;
use serde_json::{json, Value};

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let client = reqwest::Client::new();

    let response = client
        .post("https://api.emailit.com/v2/audiences/aud_1234567890/subscribers")
        .json(&json!({
            "email": "john.doe@example.com",
            "first_name": "Іван",
            "last_name": "Петренко",
            "custom_fields": {
                "company": "ТОВ \"Прогрес\"",
                "role": "Розробник"
            }
        }))
        .header("Authorization", "Bearer em_test_51RxCWJ...vS00p61e0qRE")
        .header("Content-Type", "application/json")
        .send()
        .await?;

    let result: Value = response.json().await?;
    println!("{:?}", result);

    Ok(())
}

import java.net.http.HttpClient;
import java.net.http.HttpRequest;
import java.net.http.HttpResponse;
import java.net.URI;

public class AddSubscriber {
    public static void main(String[] args) throws Exception {
        HttpClient client = HttpClient.newHttpClient();

        String jsonBody = "{\"email\":\"john.doe@example.com\",\"first_name\":\"Іван\",\"last_name\":\"Петренко\",\"custom_fields\":{\"company\":\"ТОВ \\\"Прогрес\\\"\",\"role\":\"Розробник\"}}";

        HttpRequest request = HttpRequest.newBuilder()
            .uri(URI.create("https://api.emailit.com/v2/audiences/aud_1234567890/subscribers"))
            .header("Authorization", "Bearer em_test_51RxCWJ...vS00p61e0qRE")
            .header("Content-Type", "application/json")
            .POST(HttpRequest.BodyPublishers.ofString(jsonBody))
            .build();

        HttpResponse<String> response = client.send(request,
            HttpResponse.BodyHandlers.ofString());

        System.out.println(response.body());
    }
}

using System;
using System.Net.Http;
using System.Text;
using System.Threading.Tasks;

public class AddSubscriber
{
    public static async Task Main()
    {
        using (var client = new HttpClient())
        {
            client.DefaultRequestHeaders.Add("Authorization",
                "Bearer em_test_51RxCWJ...vS00p61e0qRE");

            var json = "{\"email\":\"john.doe@example.com\",\"first_name\":\"Іван\",\"last_name\":\"Петренко\",\"custom_fields\":{\"company\":\"ТОВ \\\"Прогрес\\\"\",\"role\":\"Розробник\"}}";
            var content = new StringContent(json, Encoding.UTF8, "application/json");

            var response = await client.PostAsync(
                "https://api.emailit.com/v2/audiences/aud_1234567890/subscribers", content);

            var result = await response.Content.ReadAsStringAsync();
            Console.WriteLine(result);
        }
    }
}

curl -X POST https://api.emailit.com/v2/audiences/aud_1234567890/subscribers \
  -H "Authorization: Bearer em_test_51RxCWJ...vS00p61e0qRE" \
  -H "Content-Type: application/json" \
  -d '{
    "email": "john.doe@example.com",
    "first_name": "Іван",
    "last_name": "Петренко",
    "custom_fields": {
      "company": "ТОВ \"Прогрес\"",
      "role": "Розробник"
    }
  }'

{
  "object": "subscriber",
  "id": 1234567890,
  "audience_id": 1234567890,
  "contact_id": 1234567890,
  "email": "john.doe@example.com",
  "first_name": "Іван",
  "last_name": "Петренко",
  "custom_fields": {
    "company": "ТОВ \"Прогрес\"",
    "role": "Розробник"
  },
  "subscribed": true,
  "subscribed_at": "2021-01-01T00:00:00Z",
  "unsubscribed_at": null,
  "created_at": "2021-01-01T00:00:00Z",
  "updated_at": "2021-01-01T00:00:00Z"
}

{
  "error": {
    "code": 409,
    "message": "Підписник вже існує"
  },
  "existing": {
    "object": "subscriber",
    "id": 1234567890,
    "audience_id": 1234567890,
    "contact_id": 1234567890,
    "email": "john.doe@example.com",
    "first_name": "Іван",
    "last_name": "Петренко",
    "custom_fields": {
      "company": "ТОВ \"Прогрес\"",
      "role": "Розробник"
    },
    "subscribed": true,
    "subscribed_at": "2021-01-01T00:00:00Z",
    "unsubscribed_at": null,
    "created_at": "2021-01-01T00:00:00Z",
    "updated_at": "2021-01-01T00:00:00Z"
  }
}