How to set SD card as default location for installing apps and games on HTC desire 816

Well it is going to depend on a few things. Like what OS is the phone running. If you are running anything below Marshmallow, it will not allow you to do that. No prior OSs allow for installing straight to a SD Card. So, if you haven't (and if you can), I would suggest updating the OS to Marshmallow. That phone is supposed to allow the update.

Then there are a couple other factors. One, if the phone itself will allow it. Both due to hardware needing to be up to it and the manufacturer not having put any restriction on the phone. Sadly some do.

Now, if you can upgrade to Marshmallow there is some things you will have to do to see if it will allow the use of a card that way.

1. You will have to start with an empty SD card. Why? Because the system will be formatting it a specific way and will overwrite anything on the card. So if you don't want to lose anything, copy it to a computer, or the phone itself or a cloud account.

2. Once the card is clear, you will need to set it as "Internal" on the device rather than "External". Again Marshmallow is the only Android OS that allows this.

How to turn external SD card into internal storage on Marshmallow.

1. Go to device “Settings”, then select “Storage”.
2. Select your "SD Card", then tap the “three-dot menu“ (top-right), now select “Settings” from in there.
3. Now select “Format as internal”, and then “Erase & Format”.
4. Your SD Card will now be formatted as internal storage.
5. Reboot your phone.

NOTE: If you don't reboot the phone, many things may not work correctly, so make sure you do.

Once it is set as "Internal" you will not be able to use the SD card on other devices. They won't read it as it will have been encrypted to work as internal storage rather than just a plain old SD card. So should you wish to move stuff off of the card to a computer or somewhere else, you would have to do it the same way you would move anything off the phone itself.

If your phone doesn't allow you to do this (either won't install Marshmallow or won't allow the setting of the card as "Internal" after updating) then your only other option would be rooting the phone. Not something I would personally be able to advise you on. However they can if you go to the "Rooting, Jailbreaking & Unlocking" forum Rooting, Jailbreaking & Unlocking.