How do you use JavaScript file system API to read and write to a file?

I want to edit a text file on my website, but I don’t have a PHP server. I want to use the file path instead of a file selector to read and write to the text file. The text file is on the website, not on the client’s computer.

I tried using the website here: https://developer.mozilla.org/en-US/docs/Web/API/File_System_API#concepts_and_usage, but it doesn’t allow me to just use a file path instead of a file selector.