This is an extension of the project started for World of Warcraft multiboxing (WowOpenBox but that applies to any kind of Windows game or app, including browser games (but still without input broadcasting/not breaking any rules).
See the installation and instructions and check the “Capture foreground window” checkbox.
Use the Game menu to select your Game and/or check the Capture using foreground window:
You can then use the (configurable) hotkey
Ctrl-Shift-C to capture whichever window you want.
After which your game name should be recorded in the Game menu and you can stick to normal Auto Capture (Options Menu).
OpenMultiboxing is known to work with:
And more… try and report your experience so we can add to this list!