add menu item to the dashboard menu

I am trying the following syntax to add menu-item on dashboard:

<?php add_menu_page( $page_title, $menu_title, $capability, $menu_slug, $function, $icon_url, $position ); ?>

But, I am confused where to add this code.

Whether to add it in my theme’s functions.php or my plugin’s functions.php ?

Or any other way to add custom menu-item on the dashboard of the wordpress logged in as admin ?