append 'Add New' button on select2

view.html

// append footer at the end of dropdown list
$('#select-id').select2('container').append('<hr style="margin:5px"><a href="javascript:void(0)" onclick="add_new_option()"><img src="images/plus.png"/> Add New</a>');

// trigger on click
function add_new_option() {
  $('#select-id').select2('container').select2('close');
  
  var new_option_val, new_option_name;
	/*
	  display form/dialog box and add to db
	*/
	
	$('#select-id').append('<option value='+new_option_val+'>'+new_option_name+'</option>');
	$('#select-id').select2('val', new_option_val);
	}
}

Check this link, this works for sure