Kentico 12 Portal Engine is built with ASP.NET Webforms. That means that the simple out of the box answer is not really. The longer answer is that with enough customization it could be possible. But it would not be straight forward. You could wrap the react js code inside of a custom web part and place it in Kentico, but you would need to handle the form submission action in a special way as Webforms wraps everything in a form already. It would be easier to create the form in the Kentico forms engine if you could. Then you would get the full power and functionality of Kentico's form engine.