Vanilla JavaScript XMLHttpRequest - How to set Header correctly (Example: Pastebin Api)

Question

I just want to send a POST Request to the pastebin API and post a paste. (Vanilla JS) My problem is, that this error accours when sending the request, even if I set the header like below: "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." I get the input from the index.html which I then put into the body. (For anyone who isnt familiar with the pastebin api: https://pastebin.com/doc_api)

My index.html:

<script src="hello.js"></script>
<input id="input" type="text" placeholder="Paste Text" />
<button onclick="hello()">Send</button>

My script.js:

function hello() {
  let request = new XMLHttpRequest();
  var input = document.getElementById("input");
  var body = {
    api_dev_key: "--insert your private dev key--",
    api_option: "paste",
    api_paste_code: input,
  };
  request.open("POST", "https://pastebin.com/api/api_post.php");
  request.setRequestHeader("Access-Control-Allow-Origin", "*");
  request.send(body);
  request.onload = () => {
    console.log(request);
    if (request.status === 200) {
      console.log(JSON.parse(request.response));
    } else {
      console.log(`error ${request.status} ${request.statusText}`);
    }
  };
}