Edit in JSFiddle

var viewModel = {
    submitted: ko.observable(),
    doSomething: function (formElement) {
        this.submitted(formElement.text.value);
    }
};
ko.applyBindings(viewModel);

<form data-bind="submit: doSomething">
    <input type="text" name="text"/>
    <button type="submit">Submit</button>
</form>
<div data-bind="text: submitted"></div>