General Autocompletion
Basic autocompletion is built into macOS.  For example, you can type bjbk@ to get bjbk@example.com or similar.

Go to System Preferences... (From  Menu or your Dock or Spotlight)
Go to Keyboard
Select Text tab
Click the + button
Add text to replace, followed by the text you wish to replace it with.  The shortcuts work in most applications. (Tested MS Office and others on my system. OS X 10.10.x; Office 2011 v14.5.x)



Alternative to built in text replace:
aText (not free but very inexpensive) on the App Store is a great 3rd party alternative too.  Great for code completion or adding dates, times, etc.  (I am on no way affiliated with the developer, just a user.)
UPDATE:
Developer's note on the App Store version.

About Mac App Store version.
aText Mac App Store version can no longer fully work on OS X 10.11 El Capitan and later because of new App Sandboxing restrictions. This is not fixable.
If you purchased aText on Mac App Store, please switch to non-MAS (non sandboxed) version for free.