This is a Javascript problem. The OP needs to put the code into an event handler of some kind, usally "onchange" for that sort of thing, so it runs when you want it to run and not right away after it got parsed. Something along the lines of

    <form method="post" action="cgi-bin/login.cgi">
        <input type="email" name="email_address" required onchange="ch
+eckEmail()">
    </form>
    <script>        
        function checkEmail() {
                var xmlhttp = new XMLHttpRequest();
                ...
        }
    </script>
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