With no shortcut pinned to the taskbar, start emacs (runemacs.exe). Emacs starts and an icon appears on the taskbar.
Right-click on that icon and select "Pin this program to the taskbar". Then when you exit Emacs, the icon remains on the taskbar.
It is "pinned" (and a shortcut is created for it in the directory mentioned before).
Now when you click on the icon in the taskbar it starts Emacs. But the misbehavior is that *another* icon appears on the taskbar. One for the pinned shortcut and one for the running process. When it works, just the pinned icon on the taskbar is highlighted and multiple instances of running emacs are "grouped" as multiple icons on the taskbar. Now, setting the app id on the shortcut (and changing its target to runemacs) produces the desired behavior.