In the dashboard config file, you must add this configuration in the tiles key. The ids should contain any calendar id that you want to display on the dashboard.
In app\Console\Kernel.php you should schedule the Spatie\CalendarTile\FetchCalendarEventsCommand to run. You can let in run every minute if you want. You could also run this less frequently if fast updates on the dashboard aren't that important for this tile.
// in app/console/Kernel.phpprotectedfunctionschedule(Schedule $schedule)
{
// ...$schedule->command(Spatie\CalendarTile\FetchCalendarEventsCommand::class)->everyMinute();
}