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Pre-requisites
  • Write access to a Snapchain instance
  • Private key of an account key registered to an fid
  • An Ethereum provider URL for OP Mainnet (e.g. via Alchemy, Infura or QuickNode).
To create a Verification proving ownership of an external Ethereum address, you can use an Eip712Signer to sign a verification message, and the makeVerificationAddEthAddress function to construct a message to send to a Hub. First, set up clients and account keys: 
import {
  NobleEd25519Signer,
  ViemLocalEip712Signer,
  FarcasterNetwork,
  makeVerificationAddEthAddress,
} from '@farcaster/hub-nodejs';
import { createPublicClient, http } from 'viem';
import { privateKeyToAccount } from 'viem/accounts';
import { optimism } from 'viem/chains';

const publicClient = createPublicClient({
  chain: optimism,
  transport: http(),
});

const alice = privateKeyToAccount('0xab..12');
const eip712Signer = new ViemLocalEip712Signer(alice);

const ed25519Signer = new NobleEd25519Signer('0xcd..34');
Then create a verification signature using an Eip712Signer. You’ll need to query the latest blockhash on OP mainnet and include it in the signed message. 
const latestBlock = await publicClient.getBlock();

const fid = 1n;
const network = FarcasterNetwork.MAINNET;
const address = alice.address;
const blockHash = latestBlock.hash;

const ethSignature = await eip712Signer.signVerificationEthAddressClaim({
  fid,
  address,
  network,
  blockHash,
});
Finally, use makeVerificationAddEthAddress to construct a Verification message you can send to a Hub. 
if (ethSignature.isOk()) {
  const message = await makeVerificationAddEthAddress(
    {
      address,
      blockHash,
      ethSignature: ethSignature._unsafeUnwrap(),
    },
    { fid, network },
    ed25519Signer
  );
}

​
Full code example

[@farcaster/hub-nodejs]
import {
  NobleEd25519Signer,
  ViemLocalEip712Signer,
  FarcasterNetwork,
  makeVerificationAddEthAddress,
} from '@farcaster/hub-nodejs';
import { createPublicClient, http } from 'viem';
import { privateKeyToAccount } from 'viem/accounts';
import { optimism } from 'viem/chains';

const publicClient = createPublicClient({
  chain: optimism,
  transport: http(),
});

const alice = privateKeyToAccount('0xab..12');
const eip712Signer = new ViemLocalEip712Signer(alice);

const ed25519Signer = new NobleEd25519Signer('0xcd..34');

const latestBlock = await publicClient.getBlock();

const fid = 1n;
const network = FarcasterNetwork.MAINNET;
const address = alice.address;
const blockHash = latestBlock.hash;

const ethSignature = await eip712Signer.signVerificationEthAddressClaim({
  fid,
  address,
  network,
  blockHash,
});

if (ethSignature.isOk()) {
  const message = await makeVerificationAddEthAddress(
    {
      address,
      blockHash,
      ethSignature: ethSignature._unsafeUnwrap(),
    },
    { fid, network },
    ed25519Signer
  );
}