@PIDHWND

@PIDHWND — Searches for a window created by the specified process, and returns its handle.

Syntax:
%@PIDHWND[program,flags]

program	program name or process ID; wildcards are supported
flags	bitmapped:
	1: return handle in hexadecimal

You can specify the program as:

a filename with extension, e.g. NOTEPAD.EXE
a base name, e.g. NOTEPAD; .* is assumed
a fully-qualified filename, e.g. C:\WINDOWS\SYSTEM32\NOTEPAD.EXE
a process ID in decimal, e.g. =5892
a process ID in hexadecimal, e.g. =0x1704

@PIDHWND returns the window handle (HWND) of a matching window. If no matching window is found, @PIDHWND returns 0.

Note that a process may have no windows, one window, or many windows. The window that @PIDHWND chooses may not necessarily be the same one that you would pick. To get a list of all top-level windows created by a process, use @PIDHWNDS.