You can save a file when you can give a filename. The same as you could with SAVE AS. If it's a special save you can't do it. You also can't call an external app to do it. I recommend you go with the expected flow, which is to SUBMIT the file, not save it at all. Submitting it sends it to a web or intranet server, which runs a web script, which can do what it needs to do in a controlled way. Also stands a chance of working if the user doesn't follow your instructions, and - for example - tries to fill it in a browser, or on their phone.