Dictation keyboard shortcut
The key combination you press to start or stop dictation. To choose a shortcut, choose Apple menu 
in the sidebar. (You may need to scroll down.) Go to Dictation, click the Shortcut pop-up menu, then choose a shortcut or create your own.
Tip: If the Microphone key 
is available in the row of function keys, you can press it to start or stop dictation.