title: The CORS Header 'Access-Control-Allow-Origin' is missing
body: <p>I'm trying to use webUntis'(<a href="https://api.webuntis.dk/api/docs" rel="noreferrer">docs</a>) API for a school project. For now I'm just trying to establish any kind of connection to the API.</p>
  
  <pre><code>var result;
  const url = 'https://api.webuntis.dk/api/status';
  var xhr = new XMLHttpRequest();
  
  xhr.open('GET',url, true);
  xhr.setRequestHeader('Access-Control-Allow-Origin','*');
  xhr.setRequestHeader('Content-type','application/json');
  xhr.setRequestHeader('Access-Control-Allow-Methods','GET');
  xhr.setRequestHeader('X-API-KEY', '/*API KEY*/');
  xhr.send();
  
  
  xhr.onreadystatechange = function() {
      if (this.readyState == 4 &amp;&amp; this.status == 200) {
          result = xhr.responseType;
          console.log(result);
      }
  };
  </code></pre>
  
  <p>This code produces the following error message:</p>
  
  <blockquote>
    <p>Cross-Origin request blocked: The same origin policy prohibits the
    reading of the external resource at <a href="https://api.webuntis.dk/api/status" rel="noreferrer">https://api.webuntis.dk/api/status</a>
    (Reason: CORS Header 'Access-Control-Allow-Origin' is missing).</p>
  </blockquote>
  
  <p>How may this problem be solved? Perhaps my API key is wrong?</p>
  
  <p>Disclaimer: The error message was translated from German.</p>