Error 404 sending access token to Classroom API

In the authentication process for accessing user resources through the Classroom API, I get a 404 error return right on the Ajax sending of the access token.

I have successfully completed the previous steps. That is, the token request with the granted permissions and remaining parameters, its processing to finally send it to the Classroom API and be able to access the resources.

The cause of the error is the string that represents the destination URL of the Ajax sending.

I have tried multiple strings but it always returns the 404 error. I have also been unable to find the solution by comparing it with the strings used for the Drive or Calendar API (which appear in the examples in the documentation on the Google Identity website (https://developers.google.com/identity/protocols/oauth2/javascript-implicit-flow?hl=es-419).

The Ajax submission is done in the following function:

async function trySampleRequest() {
    var params = JSON.parse(localStorage.getItem('oauth2-test-params'));
    
    if (params && params['access_token']) {
        // User authorized the request. Now, check which scopes were granted.
    
        if            
        (params['scope'].includes('https://www.googleapis.com/auth/classroom.courses')) {
            // User authorized read-only Drive activity permission.
            // Calling the APIs, etc.
            console.log("Include scope");
            var xhr = new XMLHttpRequest();
            xhr.open('GET',
                'crossDomain : true',
                'https://www.googleapis.com/classroom/v1/about?fields=user&' +
                'access_token=' + params['access_token']);
            xhr.onreadystatechange = function (e) {
                if (xhr.readyState === 4 && xhr.status === 200) {
                  
                } else if (xhr.readyState === 4 && xhr.status === 401) {
                    // Token invalid, so prompt for user permission.
                    oauth2SignIn();
                }
            };
            xhr.send(null);
        }

    }
}

The third argument of the open method of the XMLHttpRequest() object has the value:

‘https://www.googleapis.com/classroom/v1/about?fields=user&’ +
‘access_token=’ + params[‘access_token’]

The string fragment: ‘https://www.googleapis.com/classroom/v1/about?fields=user’
I have formed it by observing the string that the documentation uses in the case of the Drive API and adapting it to the values ​​that I have assumed should be those of the Classroom API.
The string fragment:

‘&’ + ‘access_token=’ + params[‘access_token’]’

It does not generate any doubt since it is the access_token parameter and is well formed.

I have tried many options. Among others the following:

‘https://www.googleapis.com/classroom/v1/about?’

‘https://classroom.googleapis.com/classroom/v1/about?’

‘https://www.googleapis.com/classroom/v1?’

‘https://classroom.googleapis.com/classroom/v1?’

None of them work.

Could someone please tell me in which help document I could find information about this?

Thanks for your help!