Well, it looks like I've gone as far as I can here. I tried looking for the source code for the driver, but can't find anything.
You haven't mentioned, but I'm suspecting that you are running Windows. If this project is for your computer only, you will need to find the source code for the driver for your OS (Windows, MAC OS/X or Linux) and try to modify that. (FTDI is not hacker friendly; you may find it easier to write a new custom driver from scratch.) If you are trying to use the project on any computer, you will need to repeat this modified/custom driver for each OS. And then, you will need to have every user add the new driver(s) to their system. Either way, it is going to be a pretty intense project (even if you HAVE written drivers before.)
If you fall back to using Notepad (or whatever,) you can use Digikeyboard to send the Windows key and the R key (to open the "Run command" field) and then send NOTEPAD and Enter. Pause or about a second for the system to launch Notepad, and then send the message you want displayed. Pause for another 10 seconds (or however long you want the message displayed,) and send Alt and F4 to close Notepad. This will only work on one OS family, but Windows is pretty popular. You can find some ideas to make things a bit easier/faster here:
https://www.sitepoint.com/community/t/is-there-a-hotkey-to-open-up-notepad/1205/3 (or search for "keyboard shortcut for notepad".) This might be the ony practical way to get what you want for this project.
Either way, let us know what you decide. If you decide to modify/create the driver, post your work here - there are people here who are WAY smarter about writing drivers than me (I miss the days of directly accessing HW, that is why I moved to microcontrollers.

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