host.cshtml

@{
    ViewData["Title"] = "Home Page";
}

<div class="row">
    <div class="col-md-8 editor">
        
        <iframe id="TXTextControl" src="http://localhost:2957/"></iframe>
        
        <input type="button" value="Load Selection" onclick="SendMessage()" />

    </div>
</div>

<script>
    function SendMessage() {
        var receiver = document.getElementById('TXTextControl').contentWindow;
        var message = `{"txtextcontrol": {"method": "loadDocument",
            "parameter": "This is some <b>HTML</b> text."
            }}`;

        receiver.postMessage(message, 'http://localhost:2957/');
    }
</script>