Custom UI rendering in Webview

Question

How can we render a custom UI in a Web application when we load the page in iOS and Android WebViews?

I tried to open a webapp in a webview like this.

guard let url = URL(string: "https://www.abcd.com/") else {
  return
}
wkWebView.load(URLRequest(url: url))

Now, if I want to add a custom button in the web page for the mobile app alone? How that can be achieved.