android studio kotlin notification body click

You need to add setContentIntent with pending intent for the MainActivity:

class Notification : BroadcastReceiver()
{
    override fun onReceive(context: Context, intent: Intent)
    {
        val activityIntent = Intent(context, MainActivity::class.java)
        val id = 0
        val pendingIntent = PendingIntent.getActivity(context, id, activityIntent, PendingIntent.FLAG_CANCEL_CURRENT)

        val notification = NotificationCompat.Builder(context, channelID)
            .setSmallIcon(R.drawable.icon_notif_white)
            .setContentTitle(intent.getStringExtra(titleExtra))
            .setContentText(intent.getStringExtra(messageExtra))
            .setContentIntent(pendingIntent)
            .build()

        val  manager = context.getSystemService(Context.NOTIFICATION_SERVICE) as NotificationManager
        manager.notify(notificationID, notification)
    }

}