Reddit API: How to Authenticate using JavaScript fetch

I'm trying to authenticate myself but always get the error 404

const authResponse =
  await (await fetch("https://reddit.com/api/v1/access_token", {
    method: "POST",
    headers: {
      "Authorization": "Basic " + btoa("CLIENT-ID:CLIENT-SECRET"),
      "user-agent": "MyApp/1.0.0",
    },
    body: JSON.stringify({
      grant_type: "password",
      username: "MY-USERNAME",
      password: "MY-PASSWORD",
    }),
  })).json();

console.log(authResponse);

I've checked the password and the username and also the client id and the client secret but I still get the error 404. What am I doing wrong? I've followed an explanation from a Reddit post. Sadly I can't use any libraries so everything has to be done with Javascript fetch().