JSFiddle - React, Tailwind, and code Playground

HTML

<script src="http://cloud.github.com/downloads/SteveSanderson/knockout/knockout-2.0.0.js"></script>
<script src="https://raw.github.com/ericmbarnard/Knockout-Validation/master/Src/knockout.validation.js"></script>
<input data-bind="value: deneme" type="text">
<br>
<input data-bind="value: bagimli" type="text">

JavaScript

ko.validation.configure({
            registerExtenders: true,
            errorMessageClass: 'error',
            messagesOnModified: true
        });



model = new function(){
    var self = this;
    self.deneme = ko.observable('').extend({required: true});
    self.bagimli = ko.observable('');
    self.deneme.subscribe(function(val){
        if (val != '1'){
            self.bagimli.extend({required: true});
        } else {
            self.bagimli.extend({required: false});
        }
    });
           
}();

ko.applyBindings(model);
model.errors = ko.validation.group(model);
model.errors.showAllMessages();