There may be times when you want a form checkbox to start out in the checked state.  You can accomplish this by pasting this script snippet at the top of the page containing the form. You'll want to make sure you are in the Text tab in the page editor. 

<script>jQuery(function() {jQuery('#mm_custom_field_13:nth-child(3)').prop('checked', true);});</script>

Please note that if you are using this checkbox as a confirmation of Terms of Service or something similar, using this option to bypass the user's need to check the box manually may not meet legal requirements for consent.