Basic authentication with header - Javascript XMLHttpRequest

I am trying to access Adyen test API that requires basic authentication credentials. https://docs.adyen.com/developers/ecommerce-integration

My credentials work when accessing the API page through browser.

But I get an 401 Unauthorized response when trying to access the API with XMLHttpRequest POST request.

Javascript Code

var url = "https://pal-test.adyen.com/pal/servlet/Payment/v25/authorise";

var username = "ws@Company.CompanyName";
var password = "J}5fJ6+?e6&lh/Zb0>r5y2W5t";
var base64Credentials = btoa(username+":"+password);

var xhttp = new XMLHttpRequest();
xhttp.open("POST", url, true);
xhttp.setRequestHeader("content-type", "application/json");
xhttp.setRequestHeader("Authorization", "Basic " + base64Credentials);

var requestParams = XXXXXXXX;
xhttp.send(requestParams);

Result