ui-select required field validation

Question

Can we have default HTML required field validation alert box to UI-Select like the one in link given.. if so how?

https://www.google.co.in/imgres?imgurl=https://i.stack.imgur.com/R3h1L.png&imgrefurl=https://stackoverflow.com/questions/18770369/how-to-set-html5-required-attribute-in-javascript&h=178&w=675&tbnid=j8M7AwOECpa80M:&docid=NL_CUnsolMwYCM&ei=HICwVtbjJouyuASina_4Dg&tbm=isch

Edit : my code is something like

<ui-select ng-model="selectedName" name="nameSelect" required>
   <ui-select-match allow-clear="true" placeholder="Select a value">
   {{$select.selected.firstname + ' ' +$select.selected.lastname }}
   </ui-select-match>
   <ui-select-choices repeat="name in names | propsFilter: {firstname: $select.search, lastname: $select.search}" >
       <div ng-bind-html="name.firstname +  ' '+name.lastname   | highlight: $select.search">
       </div>
  </ui-select-choices>
</ui-select>

with names as list of first-names and last-names of users.