Hardhat unit test showing how to load the signer's private key, arrange struct data, and sign it

SignedData-step3.js

const { assert } = require("hardhat");
const SignedData2B = artifacts.require( 'contracts/SignedData2B.sol' );

const Web3 = hre.Web3;

const accounts = [];
let owner, signedData;
contract( 'Step2b', async function( accts ){
  owner = accts.shift();
  console.info({ owner });

  accounts.push( ...accts );
  console.info( accounts );

  it( "should deploy contracts", async () => {
    signedData = await SignedData2B.new();
    console.info({ 'Step2b.address': signedData.address });
  });
})


describe("Signature Test", function(){
  it( "test balances", async () => {
    //Imagine configuring this API in Express or Vercel
    //1) load the private key during startup...
    const web3 = new Web3();
    const PRIVATE_KEY = '0xf7af023b9220a31d5b6cf1ae098c0ec9ad11a53e305ac6f344c9bddbbf876d3d';
    const signer = web3.eth.accounts.privateKeyToAccount( PRIVATE_KEY );

    //2) when a request comes in, create an object that matches our struct (@see SignedData2B.sol)
    const mintParams = {
      id: 718,
      quantity: 2,
      expires: Math.round( Date.now() / 1000 ) + 60
    };

    //3) now convert the data into the Solidity format
    //Again, @see SignedData2B.sol to ensure the properties and types match
    const data = web3.eth.abi.encodeParameter({
        'MintParams': {
          id:       'uint256',
          quantity: 'uint256',
          expires:  'uint32'
        }
      },
      mintParams
    );

    //4) Create a hash from the contract, minter, and the data (@see SignedData-2a.sol)
    const hash = Web3.utils.soliditySha3(
      { type: 'address', value: signedData.address },
      { type: 'address', value: accounts[0] },
      { type: 'bytes',   value: data },
    );

    //5) generate the signature
    const signatureObject = signer.sign(hash);
    console.log( signatureObject );
    /*
    {
      message: '0xa52e5ce8c315bd9d272cc83ba058a2a98eb1d69d0c8eb40237f08619421c05bc',
      messageHash: '0x71ab6d3d0fb95e97dbb25814d16de699850b76ec15c2570588e012a83c0e579b',
      v: '0x1b',
      r: '0x265d41ec53ecaf101316f9894ba79e671cbd70c228becd04ec4209840acbc375',
      s: '0x5f438bce316959f4d84f4607059737d2a4a501da835401718f4723db688d011b',
      signature: '0x265d41ec53ecaf101316f9894ba79e671cbd70c228becd04ec4209840acbc3755f438bce316959f4d84f4607059737d2a4a501da835401718f4723db688d011b1b'
    }
    */

    const expectedSigner = web3.eth.accounts.recover( hash, signatureObject.signature );
    assert.equal( expectedSigner, signer.address );

    //If this was an API, you would send the mintParams and signature in the API response
    //Then your browser would send them in the transaction parameters
    await signedData.presale( mintParams, signatureObject.signature, {
      from: accounts[0],
      value: Web3.utils.toWei( '0.02' )
    });
  });
});