object/nightwatch/support matches the site

public static HumanMadeObject Nightwatch_Support()
{
    var support = new HumanMadeObject()
        .WithContext()
        .WithId("https://linked.art/example/object/nightwatch/support")
        .WithLabel("Support of Night Watch")
        .WithClassifiedAs(Aat.Support, Aat.PartType)
        .WithMadeOf("canvas", "300014078");

    support.PartOf = [
        new HumanMadeObject()
            .WithId("https://linked.art/example/object/nightwatch")
            .WithLabel("Night Watch by Rembrandt")
    ];

    return support;
}

• 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.

var humanMadeObject = new HumanMadeObject
{
    Context = ["https://linked.art/ns/v1/linked-art.json"],
    Id = "https://linked.art/example/object/nightwatch/support",
    Label = "Support of Night Watch",
    MadeOf = [Aat.Canvas],
    ClassifiedAs =
    [
        new Type("http://vocab.getty.edu/aat/300014844", "Support")
        {
            ClassifiedAs = [Aat.PartType],
        },
    ],
    PartOf =
    [
        new HumanMadeObject
        {
            Id = "https://linked.art/example/object/nightwatch",
            Label = "Night Watch by Rembrandt",
        },
    ],
};

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.