Request header field authtoken is not allowed by Access-Control-Allow-Headers in preflight response

Hello Community,

Using client.js to access external REST API, I first have to get a “token” from the public server (“”)
The request header must contain “authToken : myToken”

Code below:

var request = new Request('', {
  method: 'GET',
  headers: new Headers({
    'Content-Type': 'application/xml',
    "authToken": euriciaAuth

  .then(res => {console.log(res)}
  .then(error => {console.log(error)});

My request go througth the following error: