“Agree on Terms”-checkbox change error message

I use this question "Agree on Terms"-checkbox code to add "Agree on Terms" on my PHP Wordpress Page but i need change error messagge if user not accept the checkbox.

Is possibile?

This is my code use

<form action="#" onsubmit="if(document.getElementById('agree').checked) { return true; } else { alert('Please indicate that you have read and agree to the Terms and Conditions and Privacy Policy'); return false; }">
    <input type="checkbox" required name="checkbox" value="check" id="agree" /> I have read and agree to the Terms and Conditions and Privacy Policy
    <button type="submit" class="btn" name="reg_user">Register</button>
</form>