How do I send the following API POST request with JavaScript? [closed]

I need to send the following POST request with JavaScript (preferably with fetch() method since I am most familiar with it):

 POST /login?format={format} HTTP/1.1
    Host: authservice.priaid.ch
    Authorization: Bearer {api_key}:{hashed_credentials}

I know the api_key and took care of the hashed_credentials part already. (Here is more documentation on this API in case you need it.) I just need to send the fetch request, but don’t know how (I am very new to APIs).

I tried doing the following:

fetch('authservice.priaid.ch/login?format=json', {
      method: 'POST',
      headers: {
        Authorization: `Bearer {my_api_key}:{computedHashString}`,
      },
    })
      .then(response => response.json())
      .then(data => console.log('Success:', data))
      .catch(error => console.error('Error:', error));

but it returns an error:
POST http://127.0.0.1:5500/authservice.priaid.ch/login?format=json 405 (Method Not Allowed)