object/yiadom-boakye/1 matches the site

public static HumanMadeObject Two_Accession_Numbers()
{
    var portrait = new HumanMadeObject()
        .WithContext()
        .WithId("https://linked.art/example/object/yiadom-boakye/1")
        .WithLabel("Portrait of Lynette Yiadom-Boakye")
        .WithClassifiedAs(Aat.Painting, Aat.TypeOfWork);

    var yuagId = new Identifier("2021.25.1").AsAccessionNumber();
    yuagId.AssignedBy = [
        new AttributeAssignment()
        {
            CarriedOutBy = [
                new Group()
                    .WithId("https://linked.art/example/group/yuag")
                    .WithLabel("Yale University Art Gallery")
                    .WithClassifiedAs(Aat.Museum)
            ]
        }
    ];

    var ycbaId = new Identifier("B2021.5").AsAccessionNumber();
    ycbaId.AssignedBy = [
        new AttributeAssignment()
        {
            CarriedOutBy = [
                new Group()
                    .WithId("https://linked.art/example/group/ycba")
                    .WithLabel("Yale Center for British Art")
                    .WithClassifiedAs(Aat.Museum)
            ]
        }
    ];
    portrait.IdentifiedBy = [ yuagId, ycbaId ];

    return portrait;
}

• 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.
• Role shortcuts AsPrimaryName, AsAccessionNumber and AsSystemAssignedNumber each add a single classification, with the AAT terms Primary Name, Accession Number and System Number.

var humanMadeObject = new HumanMadeObject
{
    Context = ["https://linked.art/ns/v1/linked-art.json"],
    Id = "https://linked.art/example/object/yiadom-boakye/1",
    Label = "Portrait of Lynette Yiadom-Boakye",
    ClassifiedAs =
    [
        new Type("http://vocab.getty.edu/aat/300033618", "Painting")
        {
            ClassifiedAs = [Aat.TypeOfWork],
        },
    ],
    IdentifiedBy =
    [
        new Identifier("2021.25.1")
        {
            ClassifiedAs = [Aat.AccessionNumber],
            AssignedBy =
            [
                new AttributeAssignment
                {
                    CarriedOutBy =
                    [
                        new Group
                        {
                            Id = "https://linked.art/example/group/yuag",
                            Label = "Yale University Art Gallery",
                            ClassifiedAs = [Aat.Museum],
                        },
                    ],
                },
            ],
        },
        new Identifier("B2021.5")
        {
            ClassifiedAs = [Aat.AccessionNumber],
            AssignedBy =
            [
                new AttributeAssignment
                {
                    CarriedOutBy =
                    [
                        new Group
                        {
                            Id = "https://linked.art/example/group/ycba",
                            Label = "Yale Center for British Art",
                            ClassifiedAs = [Aat.Museum],
                        },
                    ],
                },
            ],
        },
    ],
};

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.