public class Order {
    public string Id { get; set; }
    public ICollection<OrderItem> Items { get; private set; }
}

public class Order {
    public string Id { get; set; }
    [UseDestinationValue]
    public ICollection<OrderItem> Items { get; } = new List<OrderItem>();
}

TypeAdapterConfig.GlobalSettings.Default
    .UseDestinationValue(member => member.SetterModifier == AccessModifier.None &&
                                   member.Type.IsGenericType &&
                                   member.Type.GetGenericTypeDefinition() == typeof(ICollection<>));