Despite the tooltip stating
Note that this box is free-text input and can also be any combination of replacement tokens.
The "Stop a Process by Name" action does not appear to accept tokens.
Testing with the "{WINDOWPROCESSUNDERMOUSE}", or a "{TXT:}" token where the text value is set either to the "{WINDOWPROCESSUNDERMOUSE}" token, or a process name directly, does not produce any output in the log(though, when a manually entered process name is not found, no log entry is produced either), nor is the given process terminated. If a process name is manually entered, the action terminates the process and displays (E.G.)"Stopped process notepad" in the log.
EDIT: Working in v1.6.6.9