curl --request POST \
  --url https://panel.sendcloud.sc/api/v3/contracts \
  --header 'Authorization: Basic <encoded-value>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "contract_data": {
    "country": "NL",
    "client_id": "sendcloud-1"
  },
  "carrier_code": "postnl",
  "is_active": false,
  "is_default_per_carrier": false
}
'

import requests

url = "https://panel.sendcloud.sc/api/v3/contracts"

payload = {
    "contract_data": {
        "country": "NL",
        "client_id": "sendcloud-1"
    },
    "carrier_code": "postnl",
    "is_active": False,
    "is_default_per_carrier": False
}
headers = {
    "Authorization": "Basic <encoded-value>",
    "Content-Type": "application/json"
}

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

print(response.text)

const options = {
  method: 'POST',
  headers: {Authorization: 'Basic <encoded-value>', 'Content-Type': 'application/json'},
  body: JSON.stringify({
    contract_data: {country: 'NL', client_id: 'sendcloud-1'},
    carrier_code: 'postnl',
    is_active: false,
    is_default_per_carrier: false
  })
};

fetch('https://panel.sendcloud.sc/api/v3/contracts', 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://panel.sendcloud.sc/api/v3/contracts",
  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([
    'contract_data' => [
        'country' => 'NL',
        'client_id' => 'sendcloud-1'
    ],
    'carrier_code' => 'postnl',
    'is_active' => false,
    'is_default_per_carrier' => false
  ]),
  CURLOPT_HTTPHEADER => [
    "Authorization: Basic <encoded-value>",
    "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://panel.sendcloud.sc/api/v3/contracts"

	payload := strings.NewReader("{\n  \"contract_data\": {\n    \"country\": \"NL\",\n    \"client_id\": \"sendcloud-1\"\n  },\n  \"carrier_code\": \"postnl\",\n  \"is_active\": false,\n  \"is_default_per_carrier\": false\n}")

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

	req.Header.Add("Authorization", "Basic <encoded-value>")
	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://panel.sendcloud.sc/api/v3/contracts")
  .header("Authorization", "Basic <encoded-value>")
  .header("Content-Type", "application/json")
  .body("{\n  \"contract_data\": {\n    \"country\": \"NL\",\n    \"client_id\": \"sendcloud-1\"\n  },\n  \"carrier_code\": \"postnl\",\n  \"is_active\": false,\n  \"is_default_per_carrier\": false\n}")
  .asString();

require 'uri'
require 'net/http'

url = URI("https://panel.sendcloud.sc/api/v3/contracts")

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

request = Net::HTTP::Post.new(url)
request["Authorization"] = 'Basic <encoded-value>'
request["Content-Type"] = 'application/json'
request.body = "{\n  \"contract_data\": {\n    \"country\": \"NL\",\n    \"client_id\": \"sendcloud-1\"\n  },\n  \"carrier_code\": \"postnl\",\n  \"is_active\": false,\n  \"is_default_per_carrier\": false\n}"

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

{
  "data": {
    "id": 60,
    "carrier": {
      "code": "lettresuivie",
      "name": "Lettre Suivie"
    },
    "client_id": "sendcloud-1",
    "is_active": false,
    "country": "FR",
    "is_default": false
  }
}

{
  "errors": [
    {
      "detail": "This field is required.",
      "status": "400",
      "source": {
        "pointer": "/data/client_id"
      },
      "code": "required"
    }
  ]
}