I have created one sample code of the Select box , i did get the selected value from select box.
my code sample is working in WRT but not in Web apps.

var airport = new Array('New Delhi','Chennai','Bangalore', 'Goa', 'Hyderabad', 'Kolkata', 'Mumbai', 'Pune', 'Agra', 'Agatti Island');
var from="";
var from_value="";

javascript file is

function init()
var val_option="";
for(var i=0; i<airport.length;i++)
val_option=val_option+"<option value="+airport[i]+" id="+i+">"+airport[i]+"</option>";

var cre_selectbox="<table><tr><td>Destination</td><td><select id='id_value'>"+val_option+"</select></td></tr><tr><td colspan='2'><input type=submit value='submit' name='submit' onclick='javascript:getvalue()'></td></tr></table>";
mwl.insertHTML("#selectboxid", cre_selectbox);
function getvalue()
from = document.frm.id_value.selectedIndex;
from_value = document.frm.id_value.options[from].value;
var selectedvalue="<table border=1><tr><td>"+from+"</td></tr></table>";
mwl.insertHTML("#value", selectedvalue);

html file
<body onload="init();">
<form name='frm'>
<div id="selectboxid"></div>
<div id="divvalue"></div>