function isChatbox(element) {
    return !!document.evaluate('./ancestor::div[@id="pagelet_dock" or @id="pagelet_web_messenger"]', element, null, 9, null).singleNodeValue;
}

window.addEventListener('keypress', function(event) {
    var elem = document.activeElement, value;
    if( event.keyCode === 13 && isChatbox(elem) ) {
        value = elem.value;
        elem.value = "test";
    }
}, false);