Build basic transaction

Program.cs

using NBitcoin;
using NBitcoin.Policy;
using QBitNinja.Client;
using System;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
using static System.Console;

namespace HTumbleBitProofOfConcept
{
    class Program
    {
		private static readonly Network _network = Network.TestNet;
		private static readonly BitcoinExtKey _seed = new BitcoinExtKey("tprv8ZgxMBicQKsPeGSjHbcTdpBxmVvRmySiUkQFBLruRTrn2dXAtn2rqApjVUgqsFkhfJZLYy8kXgRaEgZh7M3zdxyabF1TcwoxZsgpmAnHYyH");
		private static readonly QBitNinjaClient _qBitClient = new QBitNinjaClient(_network);

		static void Main(string[] args)
        {
			BuildBasicTransactionAsync().Wait();

			ReadKey();
        }

		private static async Task BuildBasicTransactionAsync(CancellationToken ctsToken = default(CancellationToken))
		{
			ExtKey _fundingExtKey = _seed.ExtKey.Derive(0, hardened: false);
			BitcoinAddress fundingAddress = _fundingExtKey.ScriptPubKey.GetDestinationAddress(_network); // mkvRuHAv3qek4mP3ipjqFnaFXj4d2kKit3
			WriteLine($"{nameof(fundingAddress)}: {fundingAddress}");
			uint256 txIdToSpend = (await _qBitClient.GetBalance(fundingAddress, unspentOnly: false).ConfigureAwait(false)).Operations.First().TransactionId;
			ctsToken.ThrowIfCancellationRequested();
			Transaction txToSpend = (await _qBitClient.GetTransaction(txIdToSpend).ConfigureAwait(false)).Transaction;
			ctsToken.ThrowIfCancellationRequested();

			
			ExtKey destinationExtKey = _seed.ExtKey.Derive(1, false);
			BitcoinAddress destinationAddress = destinationExtKey.ScriptPubKey.GetDestinationAddress(_network); // mk1soVb7Se1t99v7APGqiXofr2pKVS7hN5
			WriteLine($"{nameof(destinationAddress)}: {destinationAddress}");

			var fee = new Money(0.001m, MoneyUnit.BTC);
			var builder = new TransactionBuilder();
			Transaction txThatSpends = builder
				.AddCoins(txToSpend)
				.AddKeys(_fundingExtKey)
				.SendFees(fee)
				.Send(destinationAddress, txToSpend.Outputs.First().Value - fee)
				.BuildTransaction(sign: true);

			TransactionPolicyError[] errors = builder.Check(txThatSpends);
			ReportIfErrors(txName: nameof(txThatSpends), errors: builder.Check(txThatSpends));
			if (errors.Count() == 0) ReportTransaction(txName: nameof(txThatSpends), transaction: txThatSpends);
		}
	}
}