I'm trying to make my comboBox type required, but this didn't work. I can't find nothing about this. Do you know how can I achieve this?

This is the code that I tried:

<m:ComboBox id="comboId" required="true" placeholder="{i18n>ComboText}" items="{ path: 'model>/PeopleSet' }" selectionChange="onSelect" visible="false">
     <c:ListItem key="{model>Bed}" text="{model>Name}"/>
</m:ComboBox>