How to Add a Memo to a Transaction

import {
  Connection,
  Keypair,
  SystemProgram,
  LAMPORTS_PER_SOL,
  PublicKey,
  Transaction,
  TransactionInstruction,
  sendAndConfirmTransaction,
} from "@solana/web3.js";
 
(async () => {
  const fromKeypair = Keypair.generate();
  const toKeypair = Keypair.generate();
 
  const connection = new Connection(
    "https://api.devnet.solana.com",
    "confirmed",
  );
 
  const airdropSignature = await connection.requestAirdrop(
    fromKeypair.publicKey,
    LAMPORTS_PER_SOL,
  );
 
  await connection.confirmTransaction(airdropSignature);
 
  const lamportsToSend = 10;
 
  const transferTransaction = new Transaction().add(
    SystemProgram.transfer({
      fromPubkey: fromKeypair.publicKey,
      toPubkey: toKeypair.publicKey,
      lamports: lamportsToSend,
    }),
  );
 
  transferTransaction.add(
    new TransactionInstruction({
      keys: [
        { pubkey: fromKeypair.publicKey, isSigner: true, isWritable: true },
      ],
      data: Buffer.from("Memo message to send in this transaction", "utf-8"),
      programId: new PublicKey("MemoSq4gqABAXKb96qnH8TysNcWxMyWCqXgDLGmfcHr"),
    }),
  );
 
  await sendAndConfirmTransaction(connection, transferTransaction, [
    fromKeypair,
  ]);
})();