Параметры запроса#

 1curl -X POST https://test.travel-api.ru/avia/ticket_refund \
 2  -H "Authorization: Bearer YOUR_TOKEN" \
 3  -H "Content-Type: application/json" \
 4  -d '{
 5    "bookingId": "019a0ce6-2633-787f-a6bc-5dbddc02cc1f",
 6    "ticketNumber": "425-6170037294",
 7    "forRefund": {
 8      "currencyCode": "RUB",
 9      "value": 5499
10    }
11  }'

 1var request = new
 2{
 3    bookingId = "019a0ce6-2633-787f-a6bc-5dbddc02cc1f",
 4    ticketNumber = "425-6170037294",
 5    forRefund = new
 6    {
 7        currencyCode = "RUB",
 8        value = 5499
 9    }
10};
11
12var response = await httpClient.PostAsJsonAsync(
13    "https://test.travel-api.ru/avia/ticket_refund",
14    request
15);
16
17if (response.IsSuccessStatusCode)
18{
19    var content = await response.Content.ReadAsStringAsync();
20
21    // Пустой ответ {} означает успех
22    if (content.Trim() == "{}")
23    {
24        Console.WriteLine("Возврат билета успешно оформлен");
25    }
26}