To add a new option to the
select2 element you would use
tagging
Dynamic option creation | Select2 - The jQuery replacement for select boxes[
^]
Now if this is going to be a permanent addition you have an issue; as your controller is using a hard-coded list of countries to populate this box, and you can't just add to this code.
Perhaps you could move your countries to a fixed data source (e.g. SQL, XML, or a text file) which has the IDs, Text, and an "active" bit; just populate the displayed list with only those countries which are active.
And if a different country is submitted, you could update the active bit of the fixed data source for future usage