That model is running Android 6.0 and will allow you to set the SdCard as Portable storage or Internal. You need Portable to do what you want but it is a once only choice. If you nominate Portable, you lose the facility to install Apps on to the card.
When you've made that choice, make a new folder on teh card and name it Downloads. The first download you get will prompt a message as to where to store it and you can point to that folder.