You can add the value of the search text field as an option when there is a change in the search field. In order to achieve what I said, you will have to mess up the source code as I don't see any event related to the search text field on github. http://ivaynberg.imtqy.com/select2/
If you can change your preference from the drop-down list to the check box, just check out the example named "auto tokenization" on the same page.
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