The Windows API function GetCommandLine() will provide you with the complete command line to your application. Note: you will need to make a copy of the returned string, and parse out the executable path and name from the beginning of the returned string.

Example:

procedure TForm .Button Click(Sender: TObject);
var
  s : string;
begin
  s:= GetCommandLine;
  Delete(s,  , pos('exe"', s) + 4);
  Memo .Lines.Add(s);
end;