For this specific scenery, you can use something like this: (see http://jsfiddle.net/QSMv5/2/)
$( ".custom-combobox-input" ).on( "autocompleteselect", function() {
alert('hi')
} );
But, note that .custom-combobox-input is a class used in the input element of this example. On your real code, this may be different.