To achieve that example with BSelect widget you can access BSelect client-side object using its PHP name and set the on change event like this:

<script>
    functoin BSelectChanged() {
        //copy the on change function code here 
        var msg = $("#msg");
        ...        
    }
</script>
<?php
    BSelect::create(array(
        "name" => "multipleBSelect",
        ...
        "onReady" => "function() {
            multipleBSelect.on('change', BSelectChanged); // multipleBSelect is BSelect js object
        }"
    ));