I made this modification and it works for me. Let me know it this is correct.
At makeForm function of class.php file, change the code:
$onClick = 'document.getElementById(\'' . $uniqueId . '\').value = \'' . $pp[0] . '\' + String.fromCharCode(64) + \'' . $pp[1] . '\'; document.getElementById(\'' . $uniqueId . '0xff\').value = \'' . $pp[0] . '\' + String.fromCharCode(64) + \'' . $pp[1] . '\'; jQuery(\'#form_' . $key . '\').submit(); return true;';
to:
$onClick = 'if (confirm(\'' . JText::_('AGREEMENT_TEXT') . '\')) {document.getElementById(\'' . $uniqueId . '\').value = \'' . $pp[0] . '\' + String.fromCharCode(64) + \'' . $pp[1] . '\'; document.getElementById(\'' . $uniqueId . '0xff\').value = \'' . $pp[0] . '\' + String.fromCharCode(64) + \'' . $pp[1] . '\'; jQuery(\'#form_' . $key . '\').submit(); return true;}';
Previously you need to declare the AGREEMENT_TEXT at the language files.
