provenance/nuveen_commission/1 matches the site

public static Activity Commission_Of_Painting()
{
    var commission = new Activity()
        .WithContext()
        .WithId("https://linked.art/example/provenance/nuveen_commission/1")
        .WithLabel("Commission of Painting")
        .WithClassifiedAs(Aat.ProvenanceActivity)
        .WithClassifiedAs(Aat.Term("300393199", "Commissioning"));

    var franke = new Person()
        .WithId("https://linked.art/example/person/franke")
        .WithLabel("Richard Franke");
    var dine = new Person()
        .WithId("https://linked.art/example/person/dine")
        .WithLabel("Jim Dine");

    commission.Part = [
        new Activity()
            .WithLabel("Obligation to Pay Artist")
            .WithClassifiedAs(Aat.PromiseActivity)
            .WithStatement(Aat.Description, "Promise to pay for a painting to be created by the artist")
            .WithCarriedOutBy(franke)
            .WithParticipant(dine),
        new Activity()
            .WithLabel("Obligation to Produce Painting")
            .WithClassifiedAs(Aat.PromiseActivity)
            .WithReferredToBy(
                new LinguisticObject()
                    .WithClassifiedAs(Aat.Description)
                    .WithContent("Promise to produce a painting for the commissioning party"))
            .WithCarriedOutBy(dine)
            .WithParticipant(franke)
    ];

    return commission;
}

• Statement in one call WithStatement creates the LinguisticObject, classifies it as the given type and as Brief Text, and adds it under referred_to_by.
• 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 activity = new Activity
{
    Context = ["https://linked.art/ns/v1/linked-art.json"],
    Id = "https://linked.art/example/provenance/nuveen_commission/1",
    Label = "Commission of Painting",
    ClassifiedAs = [Aat.ProvenanceActivity, Aat.Term("300393199", "Commissioning")],
    Part =
    [
        new Activity
        {
            Label = "Obligation to Pay Artist",
            ClassifiedAs = [Aat.PromiseActivity],
            CarriedOutBy =
            [
                new Person
                {
                    Id = "https://linked.art/example/person/franke",
                    Label = "Richard Franke",
                },
            ],
            ReferredToBy =
            [
                new LinguisticObject
                {
                    Content = "Promise to pay for a painting to be created by the artist",
                    ClassifiedAs =
                    [
                        new Type("http://vocab.getty.edu/aat/300435416", "Description")
                        {
                            ClassifiedAs = [Aat.Statement],
                        },
                    ],
                },
            ],
            Participant =
            [
                new Person
                {
                    Id = "https://linked.art/example/person/dine",
                    Label = "Jim Dine",
                },
            ],
        },
        new Activity
        {
            Label = "Obligation to Produce Painting",
            ClassifiedAs = [Aat.PromiseActivity],
            CarriedOutBy =
            [
                new Person
                {
                    Id = "https://linked.art/example/person/dine",
                    Label = "Jim Dine",
                },
            ],
            ReferredToBy =
            [
                new LinguisticObject
                {
                    Content = "Promise to produce a painting for the commissioning party",
                    ClassifiedAs = [Aat.Description],
                },
            ],
            Participant =
            [
                new Person
                {
                    Id = "https://linked.art/example/person/franke",
                    Label = "Richard Franke",
                },
            ],
        },
    ],
};

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.