<form>
    <label>Favorite Color: </label>
    <br>
    <input id="favColor" name="favColor" type="text">
    <input type="submit" value="Submit POST Data" id="submitPost">
</form>

document.addEventListener('DOMContentLoaded', bindButtons);

function bindButtons() {
    document.getElementById('submitPost').addEventListener('click', function(event) {
        var req = new XMLHttpRequest();
        var payload = {favColor: null};
        payload.favColor = document.getElementById('favColor').value;
        req.open("POST", "http://httpbin.org/post", false);
        req.setRequestHeader('Content-Type', 'application/json');
        req.send(JSON.stringify(payload));
        var response = JSON.parse(req.responseText);
        document.getElementById('returnedFavColor').textContent = response.favColor;
        event.preventDefault();
    })
}