you would need to select all the options in the multiple select list from which dropdown checkbox is created.
<select name="cars" id="cars" multiple>
<option value="volvo" selected>Volvo</option>
<option value="saab" selected>Saab</option>
<option value="opel" selected>Opel</option>
<option value="audi" selected>Audi</option>
</select>