Here's how to load a drop down dynamically [Fiddle].
HTML
<select id="usernameDropDown">
<option>Choose a user</option>
</select>
Javascript
var select = document.getElementById("usernameDropDown");
var unames = ["Alpha", "Bravo", "Charlie", "Delta", "Echo"];
for (var i = 0; i < unames.length; i++) {
var opt = unames[i];
var el = document.createElement("option");
el.textContent = opt;
el.value = opt;
select.appendChild(el);
}