Run the app.
If your app isn’t currently running, launch it.
Select the Chrome device from the pull down
and launch it from your IDE or,
from the command line, use flutter run -d chrome,

Get the web socket info for DevTools.
At the command line, or in the IDE,
you should see a message stating something like the following:

    
Launching lib/main.dart on Chrome in debug mode...
Building application for the web...                                11.7s
Attempting to connect to browser instance..
Debug service listening on ws://127.0.0.1:54998/pJqWWxNv92s=


    Copy the address of the debug service, shown in bold.
You will need that to launch DevTools.