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Oauth Token - Ajax

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Oauth Token - Ajax

Hello all,

I'm trying to generate an Oauth Token with Ajax

But I got an error related to CORS Policy. 

Access to XMLHttpRequest at 'https://rest.zuora.com/oauth/token' from origin 'my-site' has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource.

My request is something like this:

var details = {
    'client_id': 'my-client-id',
    'client_secret': 'my-client-secret',
    'grant_type': 'client_credentials'
};
 let promise = new Promise((resolve, reject) => {
        var expectedToken;
        var xhr = new XMLHttpRequest();
        xhr.onload = function () {
            if (xhr.status === 200) {
                var jsonData = JSON.parse(this.responseText);
                expectedToken = jsonData.access_token;
                resolve(expectedToken);
            } else {
                reject("Something went Wrong")
            }
        };

        xhr.open("POST", 'https://rest.zuora.com/oauth/token', true);
        xhr.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
        xhr.send(details);
    });

With the same information, it's working from PostMan, but with Ajax it's not working, can you please help me with this?
Thanks in advance.