Hi Calpaq, I got it. I think in this case, it's better to use some conditional check in your template. Something like this:
if ( your_is_cart_empty() ) {
echo 'Please go back and add some items';
} else {
echo do_shortcode( '[mb_frontend_form id="meta-box-id"]' );
}
It's much easier than filter the form or change the fields inside.