function onChange() {
  const password = document.querySelector('input[name=password]');
  const confirm = document.querySelector('input[name=confirm]');
  if (confirm.value === password.value) {
    confirm.setCustomValidity('');
  } else {
    confirm.setCustomValidity('Passwords do not match');
  }
}

<label>password :
  <input name="password" id="password" type="password" onkeyup='check();' />
</label>
<br>
<label>confirm password:
  <input type="password" name="confirm_password" id="confirm_password"  onkeyup='check();' /> 
  <span id='message'></span>
</label>
 Run code snippet