Plugin api : error when using fetch with post, EVEN from the UI

I’m trying to make a http query to a back-end,
i solved all the CORS issues, and it works fine when i do a GET query.
However, when i try POST, i get the following message :
'<!DOCTYPE html>\n<html lang="en">\n<head>\n<meta charset="utf-8">\n<title>Error</title>\n</head>\n<body>\n<pre>Cannot POST /</pre>\n</body>\n</html>\n'

Here is my code

const response = fetch('http://localhost:1337', {
    method: 'POST',
    headers: {	
      'Accept': 'application/json',
      'Content-Type': 'application/json'
    body: JSON.stringify(inputData)
.then(result => {

Things i have tried :

I’ve seen a bunch of unresolved issue posted around that topic,
It seems quite unclear for a lot of people, it would be nice if the documentation could go more in depth (not just showing a GET example for example), i think a lot of people might be struggling with that.

Thanks in advance for the help.