How do I tell Siri to take dictation?

You don't tell Siri to take dictation.

See: Use Voice Control on your Mac - Apple Support

You will want to activate the enhanced dictation features on Mac OS X, which allow you to dictate continuously and even use voice dictation when you’re offline:

1. Choose Apple menu () > System Preferences and click on the Keyboard icon and choose the Dictation option.

2. Activate“Enhanced Dictation” (this speeds up the dictation process) and then you will be prompted to download a 1.2 GB file.

If you don’t want to use the keyboard shortcuts to activate Siri, you can set up hands-free dictation mode on your Mac, here’s how to do that:

1. Choose Apple menu () > System Preferences and click on the Accessibility icon at the bottom and on the left menu scroll down to Dictation.

2. Click the checkbox next to “Enable the dictation keyword phrase” write the first part of the two-part phrase you want to launch Siri. For example, if you want to say, “Hey Siri,” just type in “Hey”.

3. Next, you should click on the Dictation Commands button.

4. Click the box next to “Enable Advanced Commands”.

5. Click the “+” button and you’ll see three sections. For “When I Say,” enter the second part of your command, like, “Siri” if you want to say “Hey Siri.” Leave “When using” as “Any Application.”

6. For Perform, select “Open Finder Items,” then select the Siri.app file in your Applications folder and then click done.

Here is the resource I found for dictation on iphone

https://support.apple.com/en-us/HT208343

I hope this helps!

Justin

[Link Edited by Moderator]

Just to add, you can tell Siri, "turn on Voice Control".

Even "turn off Voice Control" works with Siri so one doesn't need my custom AppleScript to do it.

In macOS 10.15.2, this does not clear up two System speech processes that continue to run. In macOS 10.15.3, these have expanded to five unneeded processes after Voice Control has quit.

VikingOSX wrote:

Even "turn off Voice Control" works with Siri so one doesn't need my custom AppleScript to do it.

Yes, I know. :)

In macOS 10.15.2, this does not clear up two System speech processes that continue to run. In macOS 10.15.3, these have expanded to five unneeded processes after Voice Control has quit.

Which ones?

ps axc | egrep -i speech

And to kill them all off, whack the parent process:

pkill -9 -x com.apple.SpeechRecognitionCore.brokerd

Thanks.

.

Should one of us tell Justin that doesn't work in Catalina?

<cough>.