I believe in Forms2.0 you'll be able to do this, or something functionally similar. But that feature won't be released for 2 months probably. So for now, You need to add a piece of javascript to a custom HTML block.
<***** type="text/javascript">
$('#RequestedNewsletter').parent().children('label').html('Please check this box to receive emails from Planon. If you chose not to check this box you will not receive any emails from Planon. We are <a target="_blank" title="Our Terms and Conditions" href="#" class="navy underline">committed to your privacy</a>.');
</*****>
Replace ***** with the word script.
Where "Requested Newsletter" is the field that you want to add the label to. Finally the bold section is the text you want displayed in the label. Here I have a link (a href) on the words "committed to your privacy".