JSFiddle - React, Tailwind, and code Playground

HTML

<form data-bind="submit: SubmitQuantity">
    Quantity: 
    <input id="quantity" type="number" data-bind="value: Quantity, valueUpdate: 'afterkeydown'"/>
</form>

CSS

.hidden_button {
    width: 0px;
    height: 0px;
    font-size: 0px;
    border: 0px;
    left: -9999px;
    padding: 0px 0px 0px 0px;
}

JavaScript

var viewModel;

function ViewModel() {
    var self = this;
    self.Quantity = ko.observable('');
    self.SubmitQuantity = function() {
        if (isNaN(self.Quantity())) {
            alert('Invalid quantity');
            return false;
        }else{
            return true; // form will be submitted
        }
        self.Quantity(undefined);        
    };
}

viewModel = new ViewModel();
ko.applyBindings(viewModel);