JSFiddle - React, Tailwind, and code Playground

HTML

<label for="car">Car</label>
	<select id="car" data-bind="options: cars, value: selectedCar, enable: checkCar"></select>
	
	<label for="other">Other</label>
	<input id="other" type="text" data-bind="value: other, valueUpdate:'afterkeydown'"/>
	
	<p>Chosen value: <span data-bind="text: chosenCar"></span></p>

JavaScript

function viewModel(){
	var self = this;
	
	self.cars = ko.observableArray(["Chrysler", "Chevrolet", "Fuck You", "Lincoln"]);
	self.other = ko.observable("");
	self.selectedCar = ko.observable("");
	
    self.chosenCar = ko.computed(function() {
		return self.other() || self.selectedCar();
    });	
	
	self.checkCar = ko.computed(function(){
		if(self.other())
			return false;
		else
			return true;
	});
}

ko.applyBindings(new viewModel());