How to pick element inside iframe using document.getElementById

Use following javascript code:

var hdnSelectedInput = document.getElementById('myIframe')
                       .contentWindow.document.getElementById('hdnSelected');

contentWindow is supported by all browsers including the older versions of IE.

Note: If the iframe's src is from another domain, you won't be able to access its content due to the Same Origin Policy. 

var x = document.getElementById("myIframe");
var y = (x.contentWindow || x.contentDocument);
if (y.document)
    y = y.document;
var hdnSelectedInput = y.getElementById('hdnSelected');

The contentDocument property returns the Document object generated by a frame or iframe element.