For some reason Microsoft Word 2016 on the Mac does not have a default keyboard shortcut for footnotes. I find it tedious to take my hands off the keyboard, click insert > footnote (yes, it only takes a couple seconds but it really breaks up the flow of writing).
There are a couple solutions to this. The first is to create a keyboard shortcut on the Mac (see steps below). You can create a keyboard shortcut for any app doing this method. The second method is to use Keyboard Maestro, which is a powerful program for creating keyboard shortcuts for just about anything. The nice thing about this app is you can simulate several steps of a process with one keyboard shortcut. I prefer this method most of the time when creating keyboard shortcuts. In Microsoft Word when you go to insert a footnote it then brings up a dialogue box every time asking you how you want it formatted. This is just one more step in the process that the default Mac method cannot simulate. Using Keyboard Maestro I can insert a footnote and click OK on the next dialogue box all with one shortcut. You can download the shortcut here: http://d.pr/f/1ccMh. And here is a screenshot of what it looks like in Keyboard Maestro:
Default Mac Method
Open System Preferences (Applications > System Preferences) and click on Keyboard
Go to App Shortcuts then click the + sign
Next to Application: find Microsoft Word
Next to Menu Title: type Footnote… (you must include the three periods)
Click in the Keyboard Shortcut: box and set your shortcut