Title taken from here. For my example I did it with input.

Html:

<!DOCTYPE html>
<html>

	<head>
		<script type="text/javascript" src="/knockout-3.2.0.js"></script>
		<script type="text/javascript" src="/index.js"></script>
	</head>

	<body onload="ko.applyBindings(viewModel)">
		<input type="checkbox" placeholder="I am a test" data-bind="click: inputClick"></input>
	</body> 

</html>

JS:

var viewModel = {
	inputClick: function() {
		alert("just clicked input");
                return true;
	}
}

Here note return true. Without that state will not change (i.e. if for input you write type="checkbox")

Download from here.