''' using the requests python package, install using pip install requests '''
import requests

url = "https://api2.appsflyer.com/inappevent/[Insertapp ID here]"

payload = "{\n\"appsflyer_id\": \"9999999999999999999999\",\n\"eventName\": \"test_purchase_cancel\",\n\"eventCurrency\": \"KRW\",\n\"eventValue\": \"{\\\"af_revenue\\\": \\\"-33000\\\"}\"\n}\n"
headers = {
		'authentication': '[Insert dev key]',
		'Content-Type': 'application/json'
}

response = requests.request("POST",url, headers=headers, data = payload)

print(response.text.encode('utf8'))

/* using the request npm package, install using npm install request */
var request = require("request");

var options = { method: 'POST',
 url: 'https://api2.appsflyer.com/inappevent/<APP_ID>',
 headers: 
  { 
   "authentication": '<YOUR_DEV_KEY>',
   'Content-Type': 'application/json' 
  },
 body: 
  { appsflyer_id: '<APPS_FLYER_ID>',
   customer_user_id: '123456',
   eventName: 'node_js',
   eventValue: '{"node_js":"6" ,"af_content_id":"15854"}',
   eventCurrency: 'USD',
   ip: '1.0.0.0',
   eventTime: '2018-07-09 4:17:00.000'
   },
 json: true };

request(options, function (error, response, body) {
 if (error) throw new Error(error);

 console.log(body);
});

/* using the RestSharp package, install using NuGet */
using System;
using RestSharp;
namespace CS
{
  class Event
  {
    static void Main(string[] args)
    {
      var client = new RestClient("https://api2.appsflyer.com/inappevent/<APP_ID>");
      var request = new RestRequest(Method.POST);
      request.AddHeader("authentication", "<YOUR_DEV_KEY>");
      request.AddHeader("Content-Type", "application/json");
      var body = "{\"appsflyer_id\": \"<APPS_FLYER_ID>\"," +
            "\"customer_user_id\": \"123456\"," +
            "\"eventName\": \"af_purchase\"," +
            "\"eventValue\": \"{\\\"af_revenue\\\":\\\"6\\\" ,\\\"af_content_id\\\":\\\"15854\\\"}\"," +
            "\"eventCurrency\": \"USD\"," +
            "\"eventTime\": \"2018-07-08 4:17:00.000\"
            }";
      request.AddParameter("undefined", body, ParameterType.RequestBody);
      IRestResponse response = client.Execute(request);
      // handle response by reading response.StatusCode
      Console.WriteLine(response.Content);
    }
  }
}

$purchase_event = array(
            'appsflyer_id' => <APPS_FLYER_DEVICE_ID>,
            'idfa' => <IDFA>,
            'eventCurrency' => <PURCHASE_CURRENCY>,
            'ip' => <DEVICE_ID_ADDRESS>,
            'eventTime' => date("Y-m-d H:i:s.000",time())
            );

$purchase_event['eventName'] = 'af_purchase';
$purchase_event['eventValue'] = json_encode(array('af_revenue' => <PURCHASE_REVENUE>,
                        'af_price' => <PURCHASE_PRICE>, 
                        'af_order_id' => <PURCHASE_ORDER_ID>, 
                        'af_currency' => <PURCHASE_CURRENCY>, 
                        'af_content_type' => <PURCHASE_TYPE>, 
                        'af_quantity' => <PURCHASE_QUANTITY>, 
                        'af_content' => <PRODUCT_NAME>, 
                        'af_content_id' => <PRODUCT_ID>),
                        JSON_UNESCAPED_UNICODE);

$data_string = json_encode($purchase_event);                                          
        
$ch = curl_init('https://api2.appsflyer.com/inappevent/<YOUR_APP_ID>');                                   
curl_setopt($ch, CURLOPT_CUSTOMREQUEST, "POST");  
curl_setopt($ch, CURLOPT_HEADER, true);                                 
curl_setopt($ch, CURLOPT_POSTFIELDS, $data_string);                                 
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);                                   
curl_setopt($ch, CURLOPT_HTTPHEADER, array(                                     
'Content-Type: application/json',
'authentication: <YOUR_APPS_FLYER_DEV_TOKEN>',                                    
'Content-Length: ' .strlen($data_string))
);                                                          
                                                          
$result = curl_exec($ch);

$curl = curl_init();

AppsFlyer Go script on Github