403 Error when trying to access REST API

Not sure if I'm just missing something. Trying to get to know the REST API, and I can't get past a 403 error.

The goal is for the user to click sign in, and then to load a list of their datasets. Sample here:

https://dashbi-encore.azurewebsites.net

Code:

var ADAL = new AuthenticationContext({
    instance: 'https://login.microsoftonline.com/',
    tenant: 'common', 
    clientId: 'ee4d167d-3a52-4578-b2f6-fd6fb8efb84a', 
    redirectUri: 'https://dashbi-encore.azurewebsites.net',
    callback: userSignedIn,
    popUp: true
});

function signIn() {
    ADAL.login();
}

function userSignedIn(err, token) {
    console.log('userSignedIn called');
    if (!err) {

        showWelcomeMessage();

        var trythis = "Bearer " + token;
        var request = new XMLHttpRequest();

        request.open('GET', 'https://api.powerbi.com/v1.0/myorg/datasets');

        request.setRequestHeader('Authorization', trythis);

        request.onreadystatechange = function() {
            if (this.readyState === 4) {
                console.log('Status:', this.status);
                console.log('Body:', this.responseText);
            }
        };

        request.send();
    } else {
        console.error("error: " + err);
    }
}

function showWelcomeMessage() {
    var user = ADAL.getCachedUser();
    var divWelcome = document.getElementById('WelcomeMessage');
    divWelcome.innerHTML = "Welcome " + user.profile.name;
}

I seem to do fine getting the token from AD - and I pass it through in the header of my API request as instructed - but it just won't take. I have correctly created my application and requested the proper permissions (tried via multiple different portals) .