verification_code.gs

let EMAIL = {
  'recipient': '',
  'subject': '作業繳交驗證碼',
  'body': '請在作業提交表單填入以下驗證碼：\n'
}

function getSignature(str){
  // second argument is a RSA private key in pem format, which could e generated by $ openssl genrsa 
  let sig = Utilities.computeRsaSha256Signature(str,
    "-----BEGIN PRIVATE KEY-----\nYOUR_KEY\n-----END PRIVATE KEY-----\n");
  console.log(Utilities.base64Encode(sig));

  return Utilities.base64Encode(sig)
}

function onSubmit(e){
  let reponses =  e.response.getItemResponses();
  EMAIL.recipient = reponses[0].getResponse();
  EMAIL.body += getSignature(reponses[0].getResponse());
  MailApp.sendEmail(EMAIL.recipient, EMAIL.subject, EMAIL.body);
}