How to ignore default values while serializing json with Newtonsoft.Json

I'm using Newtonsoft.Json.JsonConvert to serialize a Textbox (WinForms) into json and I want the serialization to skip properties with default values or empty arrays.

I'v tried to use NullValueHandling = NullValueHandling.Ignore in JsonSerializerSettings but is doesn't seem to affect anything.

Here is the full code sample (simplified):

JsonSerializerSettings settings = new JsonSerializerSettings()
                {
                    Formatting = Formatting.None,
                    DefaultValueHandling = DefaultValueHandling.Ignore,
                    NullValueHandling = NullValueHandling.Ignore,
                    ReferenceLoopHandling = ReferenceLoopHandling.Ignore,
                    ObjectCreationHandling = ObjectCreationHandling.Replace,
                    PreserveReferencesHandling = PreserveReferencesHandling.None,
                    ConstructorHandling = ConstructorHandling.AllowNonPublicDefaultConstructor,
                };

    string json = JsonConvert.SerializeObject(textbox, settings);

Any ideas ?