Hello. I doubt if it is possible via browser scripts, 'cause browser have a very strict permissions concerning local files. It can download new copy of it, maybe it can copy to clipboard with one key combination. But than you most probably would need to press second combination from another script from your desktop that receiving buffer content and putting it into end of(opened?) file. But that's lay beyond what is this site for.
Autohotkey is much better suited for this.
Copy text
Hi, i need simple script that copy selected text with F6 to file on desktop mix.txt, and next time F6 add selected text at the end of that file.