object/nightwatch/18 matches the site

public static HumanMadeObject Nightwatch_Signature()
{
    var nightWatch = new HumanMadeObject()
        .WithContext()
        .WithId("https://linked.art/example/object/nightwatch/18");

    nightWatch.ReferredToBy = [
        new LinguisticObject()
            .WithClassifiedAs(Aat.SignatureStatement, Aat.Statement)
            .WithContent("signature and date: ‘Rembrandt f 1642’")
    ];

    return nightWatch;
}

• 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/18",
    ReferredToBy =
    [
        new LinguisticObject
        {
            Content = "signature and date: ‘Rembrandt f 1642’",
            ClassifiedAs =
            [
                new Type("http://vocab.getty.edu/aat/300028705", "Signature")
                {
                    ClassifiedAs = [Aat.Statement],
                },
            ],
        },
    ],
};

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.