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In this part of the tutorial, we’ll dig into how transaction arguments are built, how to request a signature according to the arguments from the smart contract, and how to broadcast the signed transaction into the network.

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Building transaction arguments

Now that we’ve set up the account and derived the public key, it’s time to build the transaction arguments. For a transaction to be valid, we must attach both a nonce and a recent_block_hash - these values ensure that the transaction is unique and prevent replay attacks. Let’s fetch nonce first: 
const accessKey = await near.connection.provider.query({
  request_type: "view_access_key",
  account_id: controllableAccountId, // "controllable.testnet"
  public_key: derivedPublicKey, // derived in the previous chapter
  finality: "optimistic",
});

const nonce = accessKey.nonce;
And recent_block_hash, now it’s your turn: 
const block = await near.connection.provider.block({
  finality: "final",
});

const blockHash = block.header.hash;
Now that we have everything needed, let’s build transaction arguments: 
const transactionArgs = {
  signer_id: controllableAccountId, // "controllable.testnet"
  signer_pk: derivedPublicKey, // derived in the previous chapter
  nonce: (nonce + 1).toString(),
  block_hash: blockHash,
};

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Requesting signature from Smart Contract

After building transaction arguments, the next step is to request a signature from the smart contract. 
const adminAccount = await near.account("admin.testnet");

const outcome = await adminAccount.functionCall({
  contractId: contractId, // "broken-rock.testnet"
  methodName: "transfer_on_behalf_of",
  args: {
    args: transactionArgs,
  },
  gas: "300000000000000",
  attachedDeposit: "100000000000000000000000", // 0.1 NEAR is enough in most cases to pay MPC fee
});

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Broadcasting the transaction

With the signature in hand, we are now ready to send it to the network! 
// Get the signed transaction from the outcome
result = providers.getTransactionLastResult(outcome);
const signedTx = new Uint8Array(result);

// Send the signed transaction
const transferOutcome = await near.connection.provider.sendJsonRpc(
  "broadcast_tx_commit",
  [Buffer.from(signedTx).toString("base64")]
);

console.log(
  `https://nearblocks.io/txns/${transferOutcome.transaction_outcome.id}`
);

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Final Thoughts

With that, we’ve successfully completed the full journey of acting on behalf of another Near account securely and transferred to yourself 0.1 NEAR from a controllable account. By following this process, you now have a solid understanding of how to manage external accounts on Near using Multi-Party Computation (MPC).