Browser extension breaking event listener

Current setup is a PHP form which customers put in an iframe with a submit button. Chrome browser extensions like TopCashBack are sending message events like:

{message: 'ACK_FRAME', targetId: 'bef025c4-852a-4b55-2ced-fcf777c43091'}

The fields sent come into this function:

function postFields(msg){
    var receiver = document.getElementById('iframe-content').contentWindow;
    receiver.postMessage(msg, 'https://my.websitecom');
}

Which this eventlistener is picking up.

        if (window.addEventListener) {
            window.addEventListener("SecureMessage", receiveMessage);
        }

Thinking that I need to be checking the origin of the browser, or somehow block extension interactions with this page?