Never mind, I got it! For future reference:
I found this code snippet:
add_action( 'wpsp_before_content', function( $settings ) {
if ( 373 === $settings['list_id'] ) {
$meta = get_post_meta( get_the_ID(), 'duration', true );
if ( $meta ) {
echo '</p>This trip will take ' . $meta . ' hours<p>';
}
}
} );
But the post didn’t mention that you should change the number of the list id, and that you can find it by looking at the shortcode you’re using. Within the echo part you can add HTML.