[{"username":"demo","email":"demo@gmail.com","password":"123"}]

<form>
    Enter Username:<br>
    <input type="text" id="usernameEn" name="username"><br>
    <button id="btnGet">Save</button>
</form>
    <br><br>
<form>
    Username:<br>
    <input type="text" id="username" name="username"><br>
    Email:<br>
    <input type="text" id="email" name="email"><br><br>
    Password:<br>
    <input type="text" id="password" name="password"><br><br>
</form>

<script type="text/javascript">
    $(document).ready(function()
    {
        $("#btnGet").click(function(event)
        {
            event.preventDefault();

        $.ajax({
            type: 'GET',
            url:  "http://localhost:8080/WebServiceTest2/webresources/users/get/" + $('#usernameEn').val(),
            dataType: "json",
            success: function(data) {
                console.log(data);
                var userDetails = data;
                renderDetails(userDetails);
            },
            error: function(jqXHR, textStatus, errorThrown) {
                alert('addWine error: ' + textStatus);
            }
        });
    });
});

function renderDetails(data)
{
    $('#username').val(data.username);
    $('#email').val(data.email);
    $('#password').val(data.password);
};
</script>