Edit in JSFiddle

var controller = {
    content : ko.observable(""),
    items : ko.observableArray([]),
    add : function(){
        var item = this.content();
        this.items.push(item);
        this.content("");
    }
};
ko.applyBindings(controller);

<form action="javascript:void()"  data-bind="submit:add">
    <fieldset>
        <input type="text" data-bind="value:content" placeholder="digite um texto para adicionar"/>
        <button type="submit">+</button>
    </fieldset>
</form>
<h2>Itens</h2>
<ul data-bind="foreach:items">
    <li data-bind="text:$data"></li>
</ul>

body{
    font-family: calibri;
}
input{
    width:90%;
}
h2{
    margin-top:3px;
    color:#7698ff;
    text-shadow:0px -1px 0px rgba(0,0,0,0.3);
    border-bottom:1px solid #7698ff; 
}