hasFocus bind

<input data-bind="hasFocus: isSelected" />
<button data-bind="click: setIsSelected">Focus programmatically</button>
<span data-bind="visible: isSelected">The textbox has focus</span>

var viewModel = {
    isSelected: ko.observable(false),
    setIsSelected: function() { this.isSelected(true) }
};
ko.applyBindings(viewModel);

<p>
    Name: 
    <b data-bind="visible: !editing(), text: name, click: edit">&nbsp;</b>
    <input data-bind="visible: editing, value: name, hasFocus: editing" />
</p>
<p><em>Click the name to edit it; click elsewhere to apply changes.</em></p>

function PersonViewModel(name) {
    // Data
    this.name = ko.observable(name);
    this.editing = ko.observable(false);
         
    // Behaviors
    this.edit = function() { this.editing(true) }
}
 
ko.applyBindings(new PersonViewModel("Bert Bertington"));