Dynamic authentication problems

I'm making a website that connects to different servers to request data. On each server I have to authenticate myself, I do it this way.

$.ajax({
    dataType: 'jsonp',
    async: false,
    xhrFields: {
        withCredentials: true
    },
    url: "http://servidor_01:8080?id=capota",
    beforeSend: function (xhr) {
        xhr.setRequestHeader ('Authorization', 'Basic ' + btoa('user:pass'));
    },
});

The problem I have is that it does not accept the user and the password, I get the typical authentication window.