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 on the right, click the pop-up menu next to Shortcut, then choose a shortcut or create your own.
Tip: If 
is available in the row of function keys, you can press it to start or stop dictation.