curl --request POST \
  --url https://{instance}.my.salesforce.com/services/apexrest/v1/order \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "businessUnitId": "a0X5g000000abcDEAA",
  "customerType": "ORGANIZATION",
  "accountId": "0015g00000XyZabAAB",
  "billToContactId": "0035g00000PqRstAAA",
  "currencyIsoCode": "USD"
}
'

import requests

url = "https://{instance}.my.salesforce.com/services/apexrest/v1/order"

payload = {
    "businessUnitId": "a0X5g000000abcDEAA",
    "customerType": "ORGANIZATION",
    "accountId": "0015g00000XyZabAAB",
    "billToContactId": "0035g00000PqRstAAA",
    "currencyIsoCode": "USD"
}
headers = {
    "Authorization": "Bearer <token>",
    "Content-Type": "application/json"
}

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

print(response.text)

const options = {
  method: 'POST',
  headers: {Authorization: 'Bearer <token>', 'Content-Type': 'application/json'},
  body: JSON.stringify({
    businessUnitId: 'a0X5g000000abcDEAA',
    customerType: 'ORGANIZATION',
    accountId: '0015g00000XyZabAAB',
    billToContactId: '0035g00000PqRstAAA',
    currencyIsoCode: 'USD'
  })
};

fetch('https://{instance}.my.salesforce.com/services/apexrest/v1/order', 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://{instance}.my.salesforce.com/services/apexrest/v1/order",
  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([
    'businessUnitId' => 'a0X5g000000abcDEAA',
    'customerType' => 'ORGANIZATION',
    'accountId' => '0015g00000XyZabAAB',
    'billToContactId' => '0035g00000PqRstAAA',
    'currencyIsoCode' => 'USD'
  ]),
  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;
}

package main

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

func main() {

	url := "https://{instance}.my.salesforce.com/services/apexrest/v1/order"

	payload := strings.NewReader("{\n  \"businessUnitId\": \"a0X5g000000abcDEAA\",\n  \"customerType\": \"ORGANIZATION\",\n  \"accountId\": \"0015g00000XyZabAAB\",\n  \"billToContactId\": \"0035g00000PqRstAAA\",\n  \"currencyIsoCode\": \"USD\"\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))

}

HttpResponse<String> response = Unirest.post("https://{instance}.my.salesforce.com/services/apexrest/v1/order")
  .header("Authorization", "Bearer <token>")
  .header("Content-Type", "application/json")
  .body("{\n  \"businessUnitId\": \"a0X5g000000abcDEAA\",\n  \"customerType\": \"ORGANIZATION\",\n  \"accountId\": \"0015g00000XyZabAAB\",\n  \"billToContactId\": \"0035g00000PqRstAAA\",\n  \"currencyIsoCode\": \"USD\"\n}")
  .asString();

require 'uri'
require 'net/http'

url = URI("https://{instance}.my.salesforce.com/services/apexrest/v1/order")

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  \"businessUnitId\": \"a0X5g000000abcDEAA\",\n  \"customerType\": \"ORGANIZATION\",\n  \"accountId\": \"0015g00000XyZabAAB\",\n  \"billToContactId\": \"0035g00000PqRstAAA\",\n  \"currencyIsoCode\": \"USD\"\n}"

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

{
  "id": "8015g00000AbCdEAAQ",
  "uuid": "f6c1c6a9-1b3e-4a2f-9a55-2c1d0e8f3a11",
  "success": true
}

{
  "success": false,
  "errors": [
    "businessUnitId is required"
  ]
}