In our recent Mac Power Users episode on macOS Sierra, both Katie and I bemoaned the fact that you can't verbally trigger Siri on the Mac. It seems a no brainer to me as someone with an iMac on my desk sitting there waiting to work 24/7. Several listeners wrote in to explain that you can trigger Siri on the Mac with your voice using the Mac's accessibility features. Lifehacker has an article showing you every step to enable voice-activated Siri on your Mac. Click on the link to set it up but I will tell you that this actually involves making two separate voice commands: "Hello" and "Computer". Having used it now a few days, I find it works best if you leave a slight delay between the two words. Have fun.