How to open a new tab in Chrome in an existing/running Chrome with cmd

If chrome is your default browser, you can try this:

start "" "http://www.google.com"

It will open the URL in the existing window or start your default browser if you haven't openend one yet.

You can also set the limitation to not open the link if the browser isn't running:

tasklist /nh|findstr "chrome.exe"&&start "" "http://www.google.com"

That will open the URL only if chrome.exe has been found within the running tasks.

The command, chrome ... doesn't seem to work

'chrome' is not recognized as an internal or external command, operable program or a batch file

That is because chrome is not in your path. You can add the Chrome directory to your path (see What are PATH and other environment variables, and how can I set or use them?) or use the full path to Chrome when running it.

Example:

"C:\Program Files (x86)\Google\Chrome\Application\chrome.exe"

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Is it possible, with cmd, to open a new tab with a specific URL in a running Google Chrome?

Yes.

Example:

rem start chrome
"C:\Program Files (x86)\Google\Chrome\Application\chrome.exe"
rem open google in a tab in the already running chrome
"C:\Program Files (x86)\Google\Chrome\Application\chrome.exe" google.com