Ok, it's a little complicated since an input list is not designed to do dropdown, since it's just a self-completed input and this event is not necessary, but you could use the following logic:
Within a $(document).ready()
you can have this field automatically selected as follows:
$(document).ready(function(){
setTimeout(function(){
$("#list").focus();
$("#browsers").mousedown();
}, 1000);
});
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<input id="list" list="browsers">
<datalist id="browsers">
<option value="Internet Explorer">
<option value="Firefox">
<option value="Chrome">
<option value="Opera">
<option value="Safari">
</datalist>
This creates a click event in the input when loading the page and so it is automatically selected, but it is still selected to be displayed, however I hope it has been helpful!