The base endpoint not using a POST request does nothing.
To add a shortened URL, make a POST request to this endpoint. Example:

curl --header "Content-Type: application/json" --request POST --data '{"key":"your_secret_key","url":"https://sean.fish"}' http://localhost:8040

or to specify the path to create the shorturl on:

curl --header "Content-Type: application/json" --request POST --data '{"key":"your_secret_key","url":"https://sean.fish","hash":"short"}' http://localhost:8040

For more info see https://github.com/seanbreckenridge/no-db-static-shorturl