curl --request POST \
  --url https://api.iso8583sim.com/build \
  --header 'Content-Type: application/json' \
  --data '
{
  "fields": {
    "2": "4111111111111111",
    "3": "000000",
    "4": "000000001000"
  },
  "mti": "0100",
  "version": "1987"
}
'

import requests

url = "https://api.iso8583sim.com/build"

payload = {
    "fields": {
        "2": "4111111111111111",
        "3": "000000",
        "4": "000000001000"
    },
    "mti": "0100",
    "version": "1987"
}
headers = {"Content-Type": "application/json"}

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

print(response.text)

const options = {
  method: 'POST',
  headers: {'Content-Type': 'application/json'},
  body: JSON.stringify({
    fields: {'2': '4111111111111111', '3': '000000', '4': '000000001000'},
    mti: '0100',
    version: '1987'
  })
};

fetch('https://api.iso8583sim.com/build', options)
  .then(res => res.json())
  .then(res => console.log(res))
  .catch(err => console.error(err));

<?php

$curl = curl_init();

curl_setopt_array($curl, [
  CURLOPT_URL => "https://api.iso8583sim.com/build",
  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([
    'fields' => [
        '2' => '4111111111111111',
        '3' => '000000',
        '4' => '000000001000'
    ],
    'mti' => '0100',
    'version' => '1987'
  ]),
  CURLOPT_HTTPHEADER => [
    "Content-Type: application/json"
  ],
]);

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

curl_close($curl);

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

package main

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

func main() {

	url := "https://api.iso8583sim.com/build"

	payload := strings.NewReader("{\n  \"fields\": {\n    \"2\": \"4111111111111111\",\n    \"3\": \"000000\",\n    \"4\": \"000000001000\"\n  },\n  \"mti\": \"0100\",\n  \"version\": \"1987\"\n}")

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

	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))

}

HttpResponse<String> response = Unirest.post("https://api.iso8583sim.com/build")
  .header("Content-Type", "application/json")
  .body("{\n  \"fields\": {\n    \"2\": \"4111111111111111\",\n    \"3\": \"000000\",\n    \"4\": \"000000001000\"\n  },\n  \"mti\": \"0100\",\n  \"version\": \"1987\"\n}")
  .asString();

require 'uri'
require 'net/http'

url = URI("https://api.iso8583sim.com/build")

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

request = Net::HTTP::Post.new(url)
request["Content-Type"] = 'application/json'
request.body = "{\n  \"fields\": {\n    \"2\": \"4111111111111111\",\n    \"3\": \"000000\",\n    \"4\": \"000000001000\"\n  },\n  \"mti\": \"0100\",\n  \"version\": \"1987\"\n}"

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

{
  "length": 123,
  "message": "<string>"
}

{
  "detail": [
    {
      "loc": [
        "<string>"
      ],
      "msg": "<string>",
      "type": "<string>",
      "ctx": {},
      "input": "<unknown>"
    }
  ]
}