Mailgun failed to load URL [Help]

Hello! I have never used mailgun before! I checked out this example and read the mailgun documentation.
I configurated everything how it should be, yet it gives me an error.

Did I forgot to add something ?
Do I have to configure something in the mailgun interface ?

  send(recipient: string, subject: string, message: string) {
    var requestHeaders = new Headers();
    requestHeaders.append("Authorization", "Basic " + this.mailgunApiKey);
    requestHeaders.append("Content-Type", "application/x-www-form-urlencoded");
    this.http.request(new Request({
      method: RequestMethod.Post,
      url: "" + this.mailgunUrl + "/messages",
      body: "from=" + this.senderMail + "&to" + recipient + "&subject=" + subject + "&text=" + message,
      headers: requestHeaders
      .subscribe(success => {
        console.log("SUCCESS -> " + JSON.stringify(success));
      }, error => {
        console.log("ERROR -> " + JSON.stringify(error));

My mailgunUrl looks like this:

This is my error:

Failed to load Request header field Authorization is not allowed by Access-Control-Allow-Headers in preflight response.

feedback.ts:65 ERROR -> {"_body":{"isTrusted":true},"status":0,"ok":false,"statusText":"","headers":{},"type":3,"url":null}

Id be glad for any input!
Thanks for reading :four_leaf_clover::blush:

Does noone has an answer or an input for me ? :disappointed_relieved:
(Sorry for selfbumping)