Redirect to www domain using AWS CloudFront Lambda @ Edge. Attach this function to the Viewer Request event.

lambda @ edge

exports.handler = (event, context, callback) => {
    const request = event.Records[0].cf.request;
    
    if (request.headers['host'][0].value !== 'www.example.com') {
        /*
         * Generate HTTP redirect response with 302 status code and Location header.
         */
        const response = {
            status: '302',
            statusDescription: 'Found',
            headers: {
                location: [{
                    key: 'Location',
                    value: 'https://www.example.com',
                }],
            },
        };
        callback(null, response);
    }
    else {
        callback(null, request);
    }
};