Issue with Ajax get request for third party log in

I'm developing a simple website. When the user opens my website, I hope to kind of redirect him to a REST endpoint that somebody else wrote, and log in there, and get the json result data from that REST API. If I open the endpoint directly from my browser, it would pop up a dialog box asking for username & password. After I enter and log in, a json string would show up saying that I logged in successfully.

This is my code

$.ajax({
    url: "http://ift.tt/2a80c9Y",
    type: "get",
    cache: true,
    crossDomain: true,
    success: function(data, textStatus) {
        alert("success " + data);
    },
    error: function(data, textStatus) {
        alert("error " + data);
    }
})

I was originally having CORS issue but that has been fixed. Now when I open my index page, it will "redirect" me to the authentication url, and the same dialog box would pop up. But after I fill in my username and password, it always returns 401 authentication failed error. If I then copy the url to a new tab in my browser, it's showing the correct json string. That means my login is correct.f016 /devsandbox/sdlpl/fliu/fliu_sdlpl/ntsdlpl/3.0/packages/Admin/Web/python/sdlw

Looks like my ajax request does not have my login info cached, or somehow does not log in. Does anyone have an idea what I did wrong and how I can fix it?

Thanks!!