var URL = "https://realla.co/api/v1/listings/search";
var usr = 'api';
var psw = 'hidden';
$.ajax({
  type: "POST",
  dataType: "json",
  contentType: "application/json",
  url: URL,
  crossDomain: true,
  beforeSend: function(xhr) {
    xhr.setRequestHeader("Authorization", "Basic " + btoa(usr + ":" + psw))
  },
  success: function(result) {
    console.log('success');
  },
  error: function(req, status, err) {
    console.log('Something went wrong', status, err);
  }
});

XMLHttpRequest cannot load https://realla.co/api/v1/listings/search. Response to preflight request doesn't pass access control check: No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin 'http://dw.dev' is therefore not allowed access. The response had HTTP status code 403.