public class MyExtension : IExtension
    {
        public int field;
        public string anotherField;

        public IExtension DeepClone()
        {
            var other = new MyExtension();
            other.field = this.field;
            other.anotherField = this.anotherField;

            return other;
        }

        public void Serialize(ISerializationWriter serializationWriter)
        {
            serializationWriter.WriteProperty("Field", this.field);
            serializationWriter.WriteProperty("AnotherField", this.anotherField);
        }
    }
…
...
    RequestTelemetry requestTelemetry = new RequestTelemetry();
    requestTelemetry.Extension = new MyExtension() { field = 42, anotherField = "value" };
    client.TrackRequest(requestTelemetry);