How do I use window.postMessage() in Construct?

Hello, I'm trying to send a message to the iFrame using window.postMessage but the server doesn't seem to get my message no matter what. Can someone give some examples of how I can correctly do that?

This is what I've tried so far:

window.postMessage ("completed", "*")

window.postMessage ("completed", window.location)

window.opener.postMessage('completed');

A = document.getelementbyid(iframe)

A.contentwindow.postmessage("completed", "*")