Unexpected character encountered while parsing value in method PostAsync

Question

I am getting error when trying to send Post message

  string productJson = await Task.Run(() => JsonConvert.SerializeObject(product, Formatting.Indented));
  HttpContent content = new StringContent(productJson, Encoding.UTF8, "application/json");
  Uri uri = new Uri(string.Format(@"{0}/api/product/addProduct?token={1}", Configuration.ServerURL, _token.TokenValue));

  var response = await _client.PostAsync(uri, content);

Model of product:

[XmlRoot(ElementName = "Product")]
public class Product
{
    [JsonProperty(PropertyName = "name")]
    [XmlElement(ElementName = "Name")]
    public string Name { get; set; }
    [JsonProperty(PropertyName = "code")]
    [XmlElement(ElementName = "Code")]
    public string Code { get; set; }
    [JsonProperty(PropertyName = "ean")]
    [XmlElement(ElementName = "Ean")]
    public string Ean { get; set; }
    [JsonProperty(PropertyName = "description")]
    [XmlElement(ElementName = "Description")]
    public string Description { get; set; }
    [JsonProperty(PropertyName = "value1")]
    [XmlElement(ElementName = "Value1")]
    public string Value1 { get; set; }
    [JsonProperty(PropertyName = "value2")]
    [XmlElement(ElementName = "Value2")]
    public string Value2 { get; set; }
    [JsonProperty(PropertyName = "productExternalNumber")]
    [XmlElement(ElementName = "ProductExternalNumber")]
    public int? ProductExternalNumber { get; set; }
    [JsonIgnore]
    [XmlElement(ElementName = "ProductExternalType")]
    public int? ProductExternalType { get; set; }
    [JsonProperty(PropertyName = "id")]
    [XmlElement(ElementName = "Id")]
    public int? DefaultStageId { get; set; }
    [JsonProperty(PropertyName = "unitName")]
    [XmlElement(ElementName = "UnitName")]
    public string UnitName { get; set; }
    [JsonProperty(PropertyName = "sourceIntegrationCode")]
    [XmlElement(ElementName = "SourceIntegrationCode")]
    public string SourceIntegrationCode { get; set; }
    [JsonProperty(PropertyName = "externalStorehouseId")]
    [XmlElement(ElementName = "ExternalStorehouseId")]
    public string ExternalStorehouseId { get; set; }
}

My JSON after serialization:

{ "name": "Bruynzeel Twin Felt Tip Flamastry 20szt", "code": "BS-2020K20B", "ean": "2010000000090", "description": "", "value1": "", "value2": "", "productExternalNumber": 9, "id": 0, "unitName": "szt.", "sourceIntegrationCode": "XL", "externalStorehouseId": "2" }

I tried using Postman to send Post message and it worked.

Full error

Newtonsoft.Json.JsonReaderException: Unexpected character encountered while parsing value: A. Path '', line 0, position 0. at Newtonsoft.Json.JsonTextReader.ParseValue() at Newtonsoft.Json.JsonTextReader.Read() at Newtonsoft.Json.JsonReader.ReadForType(JsonContract contract, Boolean hasConverter) at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.Deserialize(JsonReader reader, Type objectType, Boolean checkAdditionalContent) at Newtonsoft.Json.JsonSerializer.DeserializeInternal(JsonReader reader, Type objectType) at Newtonsoft.Json.JsonConvert.DeserializeObject(String value, Type type, JsonSerializerSettings settings) at Newtonsoft.Json.JsonConvert.DeserializeObject[T](String value, JsonSerializerSettings settings) at Newtonsoft.Json.JsonConvert.DeserializeObject[T](String value) at Y.PluginPackingIntegration.Controllers.BaseIntegrationController.<>c__DisplayClass9_0.b__0() at System.Threading.Tasks.Task`1.InnerInvoke() at System.Threading.Tasks.Task.Execute()