gistfile1.html

<form data-bind="submit: addItem">
    New item:
    <input data-bind='value: itemToAdd, valueUpdate: "afterkeydown"' />
    <button type="submit" data-bind="enable: itemToAdd.length > 0">Add</button>
    <p>Your items:</p>
    <select multiple="multiple" width="50" data-bind="options: items"> </select>
</form>