I don't know the answer to your question. But there are a few things I would check if I had your problem.

First: do you need to delay before passing the password (as suggested above)? ie. after opening your process put your process to sleep(5); to give your subprogram time to start up and ask the password question?

Secondly: some programs, like passwd on linux, use tricks to ensure you can't just pass input via STDIN - this is to prevent nasty hacks that bypass security. To get around this a language called TCL/Expect is commonly used for applications that need to call passwd (the trick used is to log in again and execute the program through a login shell). That is why I suggested a Perl-equivalent, Expect. It is possible that the program you're using needs special handling like the linux passwd program.