I've found out a bit more about why the taskkill command does not always work. If there is more than one instance of the browser open, only the one that Windows perceives to be active shows up in the task list. So if the browser App is minimised or you switch focus to another browser window, it 'dissappears' from the task list and the taskkill command fails.

Unfortunately I haven't found a workaround yet!

@Russ Thanks, ythat Works!

I've found the title changes as you navigate different pages on some sites so have used a key phrase from the title and added the wildcard * to make it more robust. Your example then becomes:

taskkill /FI "WINDOWTITLE eq Trigger CMD*"

I've found that it does not work if the window happens to be minimised, which is a bit of a pain!

I have my streaming TV / Media services set up as browser apps - it keeps them separate from any other browser activity and allows opening directly in fullscreen mode etc.
I can successfully open these in TriggerCMD using shortcuts, but haven't yet found a way of closing them.

Open commands take the form:

"C:\Program Files (x86)\Microsoft\Edge\Application\msedge_proxy.exe" --profile-directory=Default --app-id=[xxx] --app-url=https://www.bbc.co.uk/iplayer --app-launch-source=4

or
"C:\Program Files\Google\Chrome\Application\chrome_proxy.exe" --profile-directory="Profile 1" --app-id=[xxx]

where [xxx] is the unique App ID in each case

Any idea what the structure of the 'Off Command' might need to look like? I've had a play with a few variations on taskkill /f /im ... but am just stabbing around in the dark!