Unfortunately you cannot use the SD card as internal storage on your phone, well unless a few things are changed.
The original OS for that device is "pre-Marshmallow", meaning it doesn't allow for saving apps, etc to the SD Card or making the card "Internal" rather than "External". Now if your phone will allow you to upgrade it to "Marshmallow", you 'may' be able to make it work. I say 'may' because some devices just will not work with the card set as internal.
If by some chance you can upgrade, and it gives you the option to set the card as "Internal" before doing so make sure you back up everything already on the SD Card as when it is formatted to work this way, it will erase everything on the card.
If the device won't let you...