Multiselect Drop Down - New Entries

I would like to be able to type in a value into the multiselect drop down box that is not in the table it’s bound to and it will save that new value to the table.

I’m attempting to come up with a tag system. So I have a table which consists of a bunch of tags (Education, Comedy, Business, Arts, etc…). The multiselect drop down is bound to that tags table. Let’s say Drama does not exist in that table. I’d like to be able to type Drama into the multiselect drop down and it adds it as a new tag to the tag table. In other words I want to give users the ability to add their own tags.

As a work-around I set up another input box and button with a message that says something like “Don’t see a tag you’re looking for? Type here to add a new tag”
It would add the tag to the tag table and then the person could select it from the multiselect drop down.