AUI select option disabled

Just add the below option in your select

<option selected disabled hidden value=''>--select brand--</option>

<aui:select label="Brand" helpMessage="Choose your brand" name="brandOrgId" >

<option selected disabled hidden value=''>--select brand--</option>

<option value='1'>1</option>

<option value='2'>2</option>

</aui:select>