GenerateGrant.js

const abi = require('ethereumjs-abi')
const BN = require('bn.js')
const Web3 = require('web3');

const account = web3.eth.accounts.privateKeyToAccount(privateKey);

let messageHash = "0x" + abi.soliditySHA3(
  ["address", "uint", "uint"],
  [new BN(userAddress, 16), new BN(String(stakeAmount), 10), new BN(String(expiryDate), 10)]
).toString("hex");

let signedMessage = account.sign(messageHash);

response.json({
  expiryDate,
  stakeAmount,
  messageHash: signedMessage.messageHash,
  signature: signedMessage.signature,
});