C# .NET Update Custom Field item on Card using Trello REST API

Question

I'm creating an app using .NET in c# to create cards on a Trello board but I can't make updates to the custom fields. I already scoured the API documentation and the internet but cannot make it work.

The code that I'm using to update the custom field value on the card is the following:

private static async Task<String> PutCustomFieldsAsync(string Url, string jsonContent)
{
    using (var httpClient = new HttpClient())
    {
        var response = await httpClient.PutAsync(Url, new StringContent(jsonContent, System.Text.Encoding.UTF8, "application/json"));
        json = await response.Content.ReadAsStringAsync();
    };

    return json;
}

The values of the variable are the following:

Url: "https://api.trello.com/1/cards/6...3fe/customField/5...bb9/item?key=4...f&token=e...6"

jsonContent: "{"Value":{"text":"test"}}"

When I set the field manually on Trello and then get the card customfields the JSON that comes out has the following data:

{
    "id": "6...c66",
    "value": {
      "text": "teste"
    },
    "idCustomField": "5...bb9",
    "idModel": "6...3fe",
    "modelType": "card"
  }

The response that I'm getting is "400 Bad Request" due to "Invalid value for custom field type".

Can anyone help me get this to work?

Thanks.