Or, rather, you can’t through the usual Siri system preferences. However, if you’re really intent on being able to start up Siri without hitting a single key or mouse click, it’s possible to do it using Sierra’s Accessibility options. Here’s how.
Say ‘hey’ to ‘Hey Siri’
1. To start with, you’ll have change the Siri keyboard command. To do this, open the Siri preferences by going to Apple menu, clicking System Preferences, and choosing the Siri option.
2. Click Keyboard and create a custom shortcut for Siri by choosing Customize from the drop-down menu, before picking a combination of keys you want to use.
3. Next, open System Preferences and go to Accessibility.
4. Scroll down to the Dictation option, and tick to Enable the dictation keyword phrase. In the box below, enter the word “Hey.”
5. Now click Dictation Commands and make sure Enable Advanced Commands is also ticked.
6. Click the + button and make a new entry called “Siri,” with the “While Using” option reading “Any application.” The bottom option, “Perform,” should correlate with the keyboard shortcut you worked out earlier.
7. Finally, click Done. If everything is working, it should now be possible to activate Siri by saying “Hey Siri” to your Mac. Simple, no?
Source : http://www.cultofmac.com