Adding new values to chosen plugin (Multiple chosen select box )

Adding new values in chosen select box if value is not exists in a list .Chosen is a jQuery Plugin, which gives some likely looking multiple selects in your html form.

In the source code (chosen.jquery.js) Go to case 13 (13 is the keycode for ‘Enter’) option in the switch block
if (this.results_showing) {
return this.result_select(evt);
}

Add following code here
$(this.form_field).append('' + $(evt.target).val() + '');
$(this.form_field).trigger('chosen:updated');
this.result_highlight = this.search_results.find('li.active-result').last();
return this.result_select(evt);

and your final code looks like this :
case 13:
evt.preventDefault();
if (this.results_showing) {
if (!this.is_multiple || this.result_highlight) {
return this.result_select(evt);
}
$(this.form_field).append('' + $(evt.target).val() + '');
$(this.form_field).trigger('chosen:updated');
this.result_highlight = this.search_results.find('li.active-result').last();
return this.result_select(evt);
}
break;