Theres not a builtin one you would have to code an alias.

alias getid {
var %i = $get($1,0)
while (%i) {
if ($get($1,%i).hwnd == $2) { return %i }
dec %i
}
}

Something along those lines, loops all of $1's open GET windows and checks for the .hwnd id, if thats it, returns %i.
An example;

on *:filercvd:*:{
echo -s - $nick $+ 's $getid($nick,$get(-1).hwnd) $+ th Window.
}

Eamonn.