To use Siri on your Mac, you simply click on the icon for the personal digital assistant. But wouldn’t it be nice if, like on an iPhone, you could simply say “Hey, Siri” to implement it? You can, though it takes a few steps. Here’s how:
Launch System Preferences.
Go to Accessibility –> Dictation.
Check the “Enable the dictation keyword phrase” option.
In the field directly below the option, replace any of the current text with “Hey.”
Select Dictation Commands.
Check the “Enable advanced commands.”
Click the “+” button to add a new command
Change everything to match the following details: When I say: Siri; While using: Any Application; Perform: Open “Siri.app”.
Under the Perform option, select Open Finder Items… and select to open Siri found inside your Applications folder
Check off Enable the dictation keyword phrase in the Dictation preference pane
Insert “Hey” into the field directly below this box.
(This how-to is based on my experiences and info on Apple's support pages — where the images sometimes come from.)