我正试图将来自transfernow.net的HTTP API集成到我的Windows窗体应用程序中，但我遇到了一个奇怪的砖墙...
我已经用我的密钥测试了postman中的API调用，它工作得很好，但是下面的C#返回了一个403禁止，你们中的任何一个天才程序员能指出我的代码中是否有什么缺陷会导致这个问题吗（我也联系了Transfernow，但预计他们不会很快响应）

static async Task<string> SendPostRequestAsync()
{
            System.Net.ServicePointManager.SecurityProtocol System.Net.SecurityProtocolType.Tls12;

            using (var httpClient = new HttpClient())
            {
                using (var request = new HttpRequestMessage(new HttpMethod("POST"), "https://api.transfernow.net/v1/transfers"))
                {
                    request.Headers.TryAddWithoutValidation("x-api-key", "xxxxxx"); //removed for security reasons but was the same as used in postman

                    request.Content = new StringContent("{\n        \"langCode\": \"en\",\n        \"toEmails\": [\"[email protected]\", \"[email protected]\"],\n        \"files\": [{\n          \"name\": \"file1.txt\",\n          \"size\": 14587\n        },{\n          \"name\": \"music.mp3\",\n          \"size\": 5496409\n        }],\n        \"message\": \"A brillant transfer content, thank you\",\n        \"subject\": \"Text and Music\",\n        \"validityStart\": \"2022-10-28T08:52:25.955Z\",\n        \"validityEnd\": \"2022-11-01T08:52:25.955Z\",\n        \"allowPreview\": true,\n        \"maxDownloads\": 7\n      }");
                    request.Content.Headers.ContentType = MediaTypeHeaderValue.Parse("application/json");

                    var response = await httpClient.SendAsync(request);
                    if (response.IsSuccessStatusCode)
                    {
                        // Return the response content as a string
                        return await response.Content.ReadAsStringAsync();
                    }
                    else
                    {
                        // Handle an error response here
                        MessageBox.Show("Error: " + response.StatusCode, "API Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
                        return null;
                    }
                }
            }
            
            
        }

如果你能帮忙的话，我将不胜感激。
我试过使用https://curl.olsh.me/转换transfernow文档中的curl代码，甚至使用postman来生成，但都没有帮助我通过这个问题。
Postman 按要求生成代码：

var client = new HttpClient();
var request = new HttpRequestMessage(HttpMethod.Post, "https://api.transfernow.net/v1/transfers");
request.Headers.Add("x-api-key", "xxx");
var content = new StringContent("{\r\n        \"langCode\": \"en\",\r\n        \"toEmails\": [\"[email protected]\", \"[email protected]\"],\r\n        \"files\": [{\r\n          \"name\": \"P:\\\\events\\\\test\\\\export\\\\DRB_0221woodlands.jpg\",\r\n          \"size\": 2843846 \r\n        },{\r\n          \"name\": \"P:\\\\events\\\\test\\\\export\\\\DRB_0222woodlands.jpg\",\r\n          \"size\": 2843846 \r\n        }],\r\n        \"message\": \"A brillant transfer content, thank you\",\r\n        \"subject\": \"Text and Music\",\r\n        \"allowPreview\": true,\r\n        \"maxDownloads\": 7\r\n      }", null, "application/json");
request.Content = content;
var response = await client.SendAsync(request);
response.EnsureSuccessStatusCode();
Console.WriteLine(await response.Content.ReadAsStringAsync());

Postman 生成的HTTP代码：

POST /v1/transfers HTTP/1.1
Host: api.transfernow.net
Content-Type: application/json
x-api-key: xxxxxx
Content-Length: 517

{
        "langCode": "en",
        "toEmails": ["[email protected]", "[email protected]"],
        "files": [{
          "name": "P:\\events\\test\\export\\DRB_0221woodlands.jpg",
          "size": 2843846 
        },{
          "name": "P:\\events\\test\\export\\DRB_0222woodlands.jpg",
          "size": 2843846 
        }],
        "message": "A brillant transfer content, thank you",
        "subject": "Text and Music",
        "allowPreview": true,
        "maxDownloads": 7
      }