Put this in your script file:
$('select').select2({
minimumResultsForSearch: -1,
placeholder: function(){
$(this).data('placeholder');
}
});
And then in HTML, add the following code:
<select data-placeholder="Your Placeholder" multiple>
<option></option> <------ this is where your placeholder data will appear
<option>Value 1</option>
<option>Value 2</option>
<option>Value 3</option>
<option>Value 4</option>
<option>Value 5</option>
</select>