Change iframe 'src' to user clicked link

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ASP.NET

Javascript

IFrame

HI

I am very new in web designing,I want to change source URL of

iframe

by user clicking.
Suppose user clicked Login link in main menu,and he see the Login form in block of site ,And also for other menu links...
here is my html code:

<li><a href="?"  id="iframe"  önclick="getURL();">Login</a></li>

and it's JavaScript code :

function getURL() {
            var urlname = document.getElementById("iframe").getAttribute("href");
            $('iframe').attr('src', urlname);

but it didn't work correctly :
first: I do't know what set

href

in

iframe

and help in Javascript code...

Thank you ...:)