I've created a Subscription Landing Page where I've added a checkbox that requires Terms and Conditions and Privacy Terms are agreed upon. This is marked as mandatory in the HTML.

However - when testing the LP, I'm able to submit the form without checking the checkbox. I've tried manipulating the Source Code to disable the 'Sign up' and not have it available until the checkbox has been checked, but this code always appear and over-rides the disable:

onClick="return NL_landingPage_submit('next')">Sign Up</a></div>

How do I create HTML code in the LP that will not allow the LP to submit without the checkbox, and where the page doesn't generate this code and over-ride?