Your question is difficult to answer without more detailed background information. Why are you looking for a shortcode? And what kind of line should it supplement?
Design tasks are normally done in the theme itself. This may have setting options for this. As you have not named the theme you are using, it is unfortunately not possible to tell you anything about this. However, you could contact the support team for your theme.
If you are using a page builder such as Elementor, the easiest way to add such a line is certainly without any programming.
How to define shortcodes is described here: https://developer.www.ads-software.com/reference/functions/add_shortcode/ – the codes are inserted in the functions.php of your own child theme or via a code snippet plugin.
