Basic API + interface to allow the editing of static copy/text

I have a web application that will use Javascript to pull it's static copy/text content from an endpoint after the page loads.

I need a service that allows a user to edit that copy text using a non techie interface.

The user could use it to enter a plaintext string like:

"Hi , thanks for signing up!"

Into a scoped field like

login.post.signUpGreeting

Any solutions?