Message not receiving from HTML

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Hello,

I have an embeded HTML form on one page.
In this form, I would like to check if the number is not less that 100.

In HTML code I put following script

<script type="text/javascript">
// When the message is coming after check
          window.onmessage = (event) => {
            if (event.data) {
              document.getElementById("alert").innerHTML = event.data;
            }
          }
        // I get the value from input with ID "somme"
          var sumToCheck  = document.getElementById("somme").value;
        // If changed, the message should be send to WIX part
          sumToCheck.addEventListener('change', function(){
    window.parent.postMessage(sumToCheck, "https://www.mysite.me");
});

In page code I put following script

$w.onReady(function () {
    // when a message is received from the HTML element
    $w("#htmlForm").onMessage( (event) => {
        console.log("Message from HTML = " + event.data);
        var sumToCheck = event.data;
        const minSum = 990;

        if (sumToCheck < minSum || typeof sumToCheck != "number" || isNaN(sumToCheck)) {
            var alertMsg = `The sum can't be less 100`;
            console.log('not good');
            // send message to the HTML element
            htmlForm_message(event);
        } else {
            console.log('it'OK');
        }
    });
});

export function htmlForm_message(event) {
    // This function was added from the Properties & Events panel. To learn more, visit http://wix.to/UcBnC-4
    // Add your code for this event here: 
    $w("#htmlForm").postMessage(`The sum can't be less 100`);
}

When I test, nothing happen… It seams that the message not arrives from HTML form… Why? I don’t understand… I do according to documentation… But… it doesn’t work
Where is the problem?
Could somebody help me?
Thanks in advance