Points to a null-terminated string that specifies the class name or is an atom that identifies the class-name string. If this parameter is an atom, it must be a global atom created by a previous call to the GlobalAddAtom function. The atom, a 16-bit value, must be placed in the low-order word of lpClassName; the high-order word must be zero.
procedure TMainForm.Button3Click(Sender: TObject);
var
hWindow: THandle;
begin
hWindow := FindWindow('Notepad', nil);
if hWindow <> 0 then
SendMessage(hWindow, WM_SYSCOMMAND, SC_MAXIMIZE, 0);