MacBook Pro 2015 install app on external drive including a cloud storage drive
Install the application to Macintosh HD (your internal drive). Then go to your Applications folder, move the applications (after they've been installed) to the external drive. Test to make sure the apps on the drive open properly. Then delete the ones in the Applications folder.

Addition: if you have a cloud storage service, you cannot install the apps onto it, but you can store some of your other documents there while you install MS Office on your internal drive.