Edit in JSFiddle

$(function() {
    var viewModel = {
        // data
        items: ko.observableArray([]),
        itemToAdd: ko.observable(""),

        // behaviors
        addItem: function() {
            this.items.push({ name: this.itemToAdd() });
            this.itemToAdd("");
        }
    };
    ko.applyBindings(viewModel);
});

<input type="text" data-bind="value: itemToAdd" />
<button data-bind="click: addItem">Add</button>
<ul>
    <!-- ko foreach: items -->
    <li><span data-bind="text: name"/></li>
    <!-- /ko -->
</ul>