حل مشکل Misused header name Make sure request headers are used with HttpRequestMessage

      Misused header name. Make sure request headers are used with HttpRequestMessage, response headers with HttpResponseMessage, and content headers with HttpContent objects.
    

      var message = new HttpRequestMessage(HttpMethod.Post, "https://example.com");
message.Headers.Add("Content-Type", "application/json");
message.Content = new StringContent("{}");
client.SendAsync(message);
    

      var message = new HttpRequestMessage(HttpMethod.Post, "https://example.com");
message.Content = new StringContent("{}", Encoding.UTF8, "application/json");
client.SendAsync(message);