actually, it's not that hard and it may be challenging.
what you need to do is add a WebSocket Server similar to the compose Websocket Page. Then add a form (check this out:
https://getbootstrap.com/docs/5.0/forms/form-control/)
and you can add the new movie details to the form and with a button click, you send everything to the server and create automatically the *.dat file according to the movie name.
take the compose page as a reference.
create the same folder under another name and try to send data from the page to the server.
you will see it is much simpler if you try it.
i had a look at the code and i can tell you that i would do several stuff different today. i wrote it a few months ago when i just started to make b4j webapps but still the app is working fine and the new feature you want to implement is not a big deal.
try it, if you get stuck ask and i will try to help. it is important to try it by yourself and get to the result.