UNy

<form id="myform">
    <input type="text"  placeholder="username" id="login-name" name="name">
    <input type="password"   placeholder="password" id="login-pass" name="password">
    <input type="button" id="formsub" value="Вход">
</form>

<div id="tester"></div>
<script>
    let myButton = document.getElementById('formsub');
    myButton.addEventListener('click',function (ev) {
        let name = document.forms['myform'].elements['login-name'].value;
        let pass = document.forms['myform'].elements['login-name'].value;

        let obj = {
            name:name,
            pass:pass
        };
        fetch('/add', {method: 'POST',body:JSON.stringify(obj),headers:{'content-type': 'application/json'}})
                .then(function (response) {
                    return response.json();
                })
                .then(function (data) {
                    let us = document.createElement('h2');
                    us.innerHTML = data.name;
                    let mydiv = document.getElementById('tester');
                    mydiv.appendChild(us);
                })
                .catch(alert);
    })
</script>