Share option: How to append into text file?
Simpletask does what you ask for. It creates a new line in a todo.txt file. 

Personally, I use Simpletask Cloudless and sync with syncthing. There is also Simpletask Nextcloud and Simpletask Dropbox. And if you like it, we should donate to the author (just found this amazing solution two days ago, so I didn't do it yet).