No – whenever a window is mapped (created) to the Xserver, the window is given a unique ID. This is usually represented hexadecimally, and you can use FvwmIdent to ascertain it, along with the program: xwininfo and even xprop.
However, interactivally, what you can do is something like this:
WindowId $[w.id] something_that_happensAssuming the window was currently focused. Using the FVWM command ‘WindowId’ is quite specific though – what makes you think you need to use it; and more importantly, what is it that you’re trying to do?
– Thomas Adam