Accepts marketing checkbox with inline form

gistfile1.js

 $('#bis-submit').on('click', function(e) {
      e.preventDefault();
      
      var email = $('#bis_email').val(),
          variant = $('.size-handle.options li.hovered').data('variant'),
          productId = BISPopover.product.id,
          options = {
            accepts_marketing: $('#notify_opt_in').is(':checked')
          };
          
     
      BISPopover.create(email, variant, productId, options).then(function(response) {
        if (response.status == 'OK') {
          $('.BIS_error').hide();
          $('.BIS_response').show();
          $('#nbody').hide();
        } else {
          // show error message...
          $('.BIS_response').hide();
          $('.BIS_error').html(response.errors.email[0]).show();
        }
      });
});

This assumes you have a checkbox with the correct id:

<input type="checkbox" id="notify_opt_in" value="1">
<label for="notify_opt_in">Add me to the newsletter</label