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);
	}
}

Couldn't get this to work. Getting an "apply" error.

This is because of the .select2('container'), try add containerCssClass option on init and select the container with the specified class

Check this link, this works for sure