<script>
    // Create the method, that the async script calls
    window.myAsyncInit = function() {
        AsyncApp.init({
            option1: 'string',
            option2: 3.14,
            option3: true
        });
    };

    // Create and append the script object
    (function(d){
        var js,
            id = 'async-script',
            ref = d.getElementsByTagName('script')[0];

        // If the script was already loaded just return
        if (d.getElementById(id)) {return;}

        // Create the script object
        js = d.createElement('script');
        js.id = id;
        js.async = true;
        js.src = "async.js";       // url to your javascript file

        // Append the script object
        ref.parentNode.insertBefore(js, ref);
    }(document));
</script>