visual/nightwatch/40 matches the site

public static VisualItem NightWatch_Style()
{
    var appearance = new VisualItem()
        .WithContext()
        .WithId("https://linked.art/example/visual/nightwatch/40")
        .WithLabel("Appearance of Night Watch")
        .WithClassifiedAs(Aat.Term("300021147", "Baroque"), Aat.Style);

    return appearance;
}

• Metatype classification The second argument classifies the first, not the object: WithClassifiedAs(Aat.Description, Aat.Statement) says this is a Description, and Description is a kind of Statement. The Plain C# form shows the nesting.
• Terms outside the registry The vocabulary registry holds only the terms linked.art itself publishes, so a dataset's own concepts, places and people are written with Aat.Term, Tgn.Place and Ulan.Person, giving the id and the label.

var visualItem = new VisualItem
{
    Context = ["https://linked.art/ns/v1/linked-art.json"],
    Id = "https://linked.art/example/visual/nightwatch/40",
    Label = "Appearance of Night Watch",
    ClassifiedAs =
    [
        new Type("http://vocab.getty.edu/aat/300021147", "Baroque")
        {
            ClassifiedAs = [Aat.Style],
        },
    ],
};

Generated from the object the example builds: every property that is set, as object initialisers, with the vocabulary constants where a term is in the registry. The fluent helpers are a convenience, not a requirement; this form and the one as written produce the same JSON.