iFrame onload JavaScript event

I have an iFrame, where I want to send a JavaScript command after loading. My current code looks like this:

<iframe src="http://www.test.tld/" onload="__doPostBack('ctl00$ctl00$bLogout','')">

But with this code the command isn't executed. What must I change to make it work? Only Chrome and Firefox have to be supported.

Use the iFrame's .onload function of JavaScript:

<iframe id="my_iframe" src="http://www.test.tld/">
    <script type="text/javascript">
        document.getElementById('my_iframe').onload = function() {
            __doPostBack('ctl00$ctl00$bLogout','');
        }
    </script>
    <!--OTHER STUFF-->
</iframe>

document.querySelector("iframe").addEventListener( "load", function(e) {

    this.style.backgroundColor = "red";
    alert(this.nodeName);

    console.log(e.target);

} );

<iframe src="example.com" ></iframe>