curl --request POST \
  --url https://api.terminal49.com/v2/shipments/{shipment_id}/custom_fields \
  --header 'Authorization: <api-key>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "data": {
    "type": "custom_field",
    "attributes": {
      "api_slug": "customer_reference_number",
      "value": "ABC124"
    }
  }
}
'

import requests

url = "https://api.terminal49.com/v2/shipments/{shipment_id}/custom_fields"

payload = { "data": {
        "type": "custom_field",
        "attributes": {
            "api_slug": "customer_reference_number",
            "value": "ABC124"
        }
    } }
headers = {
    "Authorization": "<api-key>",
    "Content-Type": "application/json"
}

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

print(response.text)

const options = {
  method: 'POST',
  headers: {Authorization: '<api-key>', 'Content-Type': 'application/json'},
  body: JSON.stringify({
    data: {
      type: 'custom_field',
      attributes: {api_slug: 'customer_reference_number', value: 'ABC124'}
    }
  })
};

fetch('https://api.terminal49.com/v2/shipments/{shipment_id}/custom_fields', 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.terminal49.com/v2/shipments/{shipment_id}/custom_fields",
  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([
    'data' => [
        'type' => 'custom_field',
        'attributes' => [
                'api_slug' => 'customer_reference_number',
                'value' => 'ABC124'
        ]
    ]
  ]),
  CURLOPT_HTTPHEADER => [
    "Authorization: <api-key>",
    "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.terminal49.com/v2/shipments/{shipment_id}/custom_fields"

	payload := strings.NewReader("{\n  \"data\": {\n    \"type\": \"custom_field\",\n    \"attributes\": {\n      \"api_slug\": \"customer_reference_number\",\n      \"value\": \"ABC124\"\n    }\n  }\n}")

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

	req.Header.Add("Authorization", "<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))

}

HttpResponse<String> response = Unirest.post("https://api.terminal49.com/v2/shipments/{shipment_id}/custom_fields")
  .header("Authorization", "<api-key>")
  .header("Content-Type", "application/json")
  .body("{\n  \"data\": {\n    \"type\": \"custom_field\",\n    \"attributes\": {\n      \"api_slug\": \"customer_reference_number\",\n      \"value\": \"ABC124\"\n    }\n  }\n}")
  .asString();

require 'uri'
require 'net/http'

url = URI("https://api.terminal49.com/v2/shipments/{shipment_id}/custom_fields")

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

request = Net::HTTP::Post.new(url)
request["Authorization"] = '<api-key>'
request["Content-Type"] = 'application/json'
request.body = "{\n  \"data\": {\n    \"type\": \"custom_field\",\n    \"attributes\": {\n      \"api_slug\": \"customer_reference_number\",\n      \"value\": \"ABC124\"\n    }\n  }\n}"

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

{
  "data": {
    "id": "3c90c3cc-0d44-4b50-8888-8dd25736052a",
    "type": "custom_field",
    "attributes": {
      "api_slug": "<string>",
      "value": "<string>",
      "display_value": "<string>"
    },
    "relationships": {
      "entity": {
        "data": {
          "type": "shipment",
          "id": "3c90c3cc-0d44-4b50-8888-8dd25736052a"
        }
      },
      "definition": {
        "data": {
          "type": "custom_field_definition",
          "id": "3c90c3cc-0d44-4b50-8888-8dd25736052a"
        }
      }
    }
  },
  "links": {
    "self": "<string>"
  }
}

{
  "errors": [
    {
      "detail": "You are not allowed to perform this action"
    }
  ]
}

{
  "errors": [
    {
      "status": "404",
      "title": "Not Found"
    }
  ]
}

{
  "errors": [
    {
      "status": "422",
      "source": {
        "pointer": "/data/attributes/label"
      },
      "title": "Unprocessable Entity",
      "detail": "Label can't be blank",
      "code": "blank"
    }
  ]
}