Well, your phone comes with Android 5 "Lollipop", which cannot do as you are seeking to (if you are wanting to make it the direct download and installation for apps. You would need to be running, or upgrade to Android 6 "Marshmallow". If you have or can upgrade to that, then it is just a matter of seeing if the steps will work. Some manufacturers restrict this option.
How to turn external SD card into internal storage on "Marshmallow".
NOTE: Please be sure you start with a blank SD card. It needs to be blank because when the card is converted to "Internal" storage, it will be formatted (wiped) and encoded (making the cards data only readable by that device).
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 tablet.
NOTE: If you don't reboot the tablet, many things may not work correctly, so make sure you do.
Now, if your phone can't either upgrade the OS or is restricted by the manufacturer, the other option you have is to move things like Photos, Videos, Music, etc., over to the SD card. They can be put there, and they also take up a lot of space.