JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://github.com/SteveSanderson/knockout/raw/master/build/output/knockout-latest.debug.js"></script>
<script src="https://github.com/SteveSanderson/knockout.mapping/raw/master/build/output/knockout.mapping-latest.debug.js"></script>
<script src="https://github.com/jquery/jquery-tmpl/raw/master/jquery.tmpl.js"></script>
<input type="text" data-bind="value: description, valueUpdate: 'afterkeydown'" />
<p data-bind="text: mydunamicfield"></p>

JavaScript

var viewModel = { 
    description:ko.observable("test"), 
    Name:ko.observable(""), 
    productid:ko.observable(""), 
    productmodel:ko.observable(""), 
    productnumber:ko.observable(""), 
    text_relevance:ko.observable("")
    }; 

    viewModel.mydunamicfield = ko.computed(function() { 
        return "bq=(and "+((this.description() == "") ? "" : ("description:"+this.description()+" ")) + ")";
    }, viewModel) 


ko.applyBindings(viewModel);