It depends on what you are actually trying to download to the card. If you are seeking to have it be where any apps go, well it depends on which version of Android your galaxy is running. If it is anything lower than "Marshmallow" then you can't set apps to download direct to the SD Card. Or move most of them for that matter, as they only work on "Internal" memory and the card is "External" memory. If your device is running "Marshmallow" then you can try the information below to see if you can set the card as "Internal". But please make sure you start with a blank SD Card as the change will format and encode the card, erasing anything that was on it.
If you don't have "Marshmallow" (or your device won't allow you to change the card to "Internal") then the only things you can move to the card would be photos, videos, music, etc. Some of those (depending on the app you do it through) will let you set up so that they download directly to the card.
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 tablet.
NOTE: If you don't reboot the tablet, many things may not work correctly, so make sure you do.