postmessage.html

<pre id='hoge'></pre>
<input id='foobar' />
<button id='send'>send</button>
<script>
const hoge = document.getElementById('hoge');
const foobar = document.getElementById('foobar');
const send = document.getElementById('send');
let target; 

if (window.opener) {
    // 子window
    target = window.opener;
} else {
    // 親window
    target = window.open(window.location, '_blank');
}
    
window.addEventListener('message', (message) => hoge.textContent += message.data + '\n');
send.addEventListener('click', () => {
    target.postMessage(foobar.value, '*');
    hoge.textContent += foobar.value + '\n';
    foobar.value = '';
});
</script>