Before we start the guide, on how to achieve this, I just would like to mention that this feature is only possible on Android 6. Also, once formatted, the sdcard will only not be usable on any other equipment.
If you attempt to boot the device without this SDcard before reverting it to its original state, it may cause some troubles.
You also must be aware that some applications will not be able to move, and depending on your SDcard, applications running on it, can become slower.
Now, that said and if you are aware of all of these downsides, please follow the steps below
While on the Home Screen, press the menu button on the remote (3 stripes button)
A menu on the left will open, and from here, select More,
Storage & Reset
Now, select your SDcard.
Now, select Format as Device Storage
Once you select that, you will get a warning of the downsides. Almost everything that is stated in the intro of this article.
If you wish to proceed, just select Format.
After this, you can start using your SDcard as Internal Storage for your media and applications.
If you do not know how to move applications to your SDcard, just follow this guide.