InstallAware

The application in question runs hidden, all the time, in the background.
I want to close down the application gracefully, by starting a duplicate of the executable with the /Uninstall parameter.
This will make all running instances close down, prior to uninstalling.
I have tested it, it works like magic, only if I knew how to run something prior to uninstalling!

How do I do this?

Run it immediately before Apply Uninstall.

It works like magic, I added it to the script.

Later I found a more obvious and much easier way:
Advanced Options / Run Programs / + New
and check "Before Uninstall"