JSFiddle - React, Tailwind, and code Playground

HTML

@{
        string pn = Request.Form["pn"];
        if (!string.IsNullOrEmpty(pn)) {    // This is NOT true the first time the user loads this page
            var db123 = Database.Open("DIALER");
            var query123 = db123.Query ("Exec sp_AddPNtoDNC " + pn );
        }
        else {
            pn = "";
        }
        <script>
            var pn = '@(pn)';
 
        </script>
        
}
 
<!DOCTYPE html>
<script>
function submitDNC(event) {
    var valid = false;

    alert('Hello world');
    // your validation logic goes here, sets valid to TRUE if it's valid

    if(!valid) {
        event.preventDefault();
    }
}
document.getElementById('addDNCform').addEventListener( 'submit', submitDNC, false );
</script>
 
<html lang="en">
    <head>
        <meta charset="utf-8" />
        <title>Add Number to DNC</title>
        <link rel="stylesheet" href="normalize.css" />
    </head>
 
    <body>
 
        <div id="header">
            <h3>Add Phone Number to Dialer Do Not Call List</h3>
        </div>
 
        <div id="add-number">
            <form  action="add-another-number-to-dnc.cshtml" method="post" id="addDNCform">
                <h4>Enter 10-digit phone number without dashes, dots or parenthesis</h4> 
                <input type="text" name="pn" required placeholder="phone number" 
                    title="Phone Number to Add to Do-Not-Call List" 
                    onkeypress="if (event.keyCode == 13) document.getElementById('btnVldt').click()"/> <!-- all this is to treat [Enter] as a click -->
                <input id="btnVldt" type="button" value="Add Number to Do Not Call list" onclick="return submitDNC()"/>
            </form>
        </div>
 
        <div id="result">         
            <script>
                if (pn.length > 0) {
                    document.getElementById("result").innerHTML= 
                    "<p><b>Phone number " + pn + " was added to the Dialer Do Not Call list.</b></p>" +
                    "<p>You can...