JavaScript - Problema con CORS en fetch

var myHeaders = new Headers();

myHeaders.append('Content-Type', 'application/json');

myHeaders.append('Accept', 'application/json');

myHeaders.append('Access-Control-Allow-Origin', 'http://localhost:8000');

myHeaders.append('Access-Control-Allow-Credentials', 'true');

myHeaders.append("x-inbenta-key", "my_key");

var raw = JSON.stringify({"secret":"secret_key"});

var requestOptions = {

    method: 'POST',

    headers: myHeaders,

    body: raw,

};

fetch("external_api_url", requestOptions)

    .then(response => response.text())

    .then(result => console.log(result))

    .catch(error => console.log('error', error));

Access to fetch at 'external_api_url' from origin 'http://127.0.0.1:8000' has been blocked by CORS policy: Response to preflight request doesn't pass access control check: No 'Access-Control-Allow-Origin' header is present on the requested resource. If an opaque response serves your needs, set the request's mode to 'no-cors' to fetch the resource with CORS disabled.