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');
}
}
<form>
<label>Password: <input name="password" type="password" onChange="onChange()" /> </label><br />
<label>Confirm : <input name="confirm" type="password" onChange="onChange()" /> </label><br />
<input type="submit" />
</form>