Edit in JSFiddle

var countries = [
    "Mauritius",
    "Sudan",
    "Uzbekistan",
    "Uganda"
];
$( "#countries" ).autocomplete({
    source: countries
}).val('u').trigger('keydown');

<h1>jQuery UI</h1>
<div class="ui-widget">
  <label for="countries">Country: </label>
    <input id="countries"/>
</div>