Use Siri to fill in input fields on macOS

How can I get Siri to fill in input fields on MacBook Pro?

At the moment I have to copy paste from the Siri dialogue.

Edit____________

I'm not interested in answers relating to normal dictation in the mac, I WANT SIRI!! )

There are a few things to consider. Siri will only work if you are connected to the internet. Dictation commands (enhanced dictation) works with or without an internet connection. Siri's capabilities are quite limited. The beauty of enabling and using enhanced dictation commands, is that you can create your own dictation commands. Getting used to the whole process takes a little trial and error. I barely ever use my mouse anymore. I've created so many new dictation commands that I can virtually use dictation commands for entering passwords into password fields, create and send iMessages, emails, opening and closing applications, etc.
In fact, this entire answer I am posting was done with dictation commands. If you really want to roll your sleeves up and dive in, you can create AppleScripts that can be very complex then the save that as an application and enable that application as a spoken dictation command.

For example, I wrote an AppleScript that takes screenshots of my computer screen if the motion sensor on my built in iSight camera gets triggered.
The screenshots are stored on my desktop and then automatically messaged to me through iMessages. Then once the message has been sent, the snapshot gets deleted from my desktop automatically and stored in my dropbox. Long story short, with dictation commands, all I need to say is "Run Surveillance" which activates the video software with the motion detection, and from there the scripts get triggered.

Here is a quick snippet of the iMessage part of the script.

set exportAs to "Screengrab.png"
set myScreengrab to POSIX file "/Users/Smokestack/Desktop/iSentry_Motion_Detector/Screengrab.png"
do shell script "screencapture -mx -T0 " & quoted form of POSIX path of myScreengrab
tell application "Messages"
    activate
    set targetBuddy to "[email protected]"
    set targetService to id of 1st service whose service type = iMessage
    set textMessage to myScreengrab
    set theBuddy to buddy targetBuddy of service id targetService
    send textMessage to theBuddy
    delay 5
    tell application "System Events"
        keystroke "h" using (command down)
    end tell
end tell
tell application "iSentry"
    activate
end tell
tell application "Finder"
    delete myScreengrab
end tell

Really there is no limit to how much of your computer you can control with dictation commands as long as you are willing to do some research and a whole lot of reading and trial and error.

To use normal dictation, focus an input field and press fn twice (default shortcut).
For more dictation settings, see System Preferences → Keyboard → Dictation.