When a premium plugin is bundled with the theme, you can use the plugin without any issues. Such plugins are fully functional even though you don’t get a plugin purchase code for activation.
Since you purchased a bundled product, you do NOT have a license key for that plugin so you can’t enter anything in this field. Updates to the plugins are provided when the theme itself is updated and via tickets.
However, if you want automatic updates and support from the plugin developer you can optionally purchase it separately.