How can I use a 3rd party keyboard with 3D WebView?

In 3D WebView, all keyboard input is sent to a webview through the IWebView.HandleKeyboardInput() method like described in this article. For example, 3D WebView's on-screen Keyboard component and its HardwareKeyboardListener script both work by emitting the string for a character so that the application can call HandleKeyboardInput(), like this:

var keyboard = Keyboard.Instantiate();
keyboard.InputReceived += (sender, eventArgs) => {
  webViewPrefab.WebView.HandleKeyboardInput(eventArgs.Value);
};

So, you can integrate with any 3rd party keyboard by passing the character output from the keyboard to HandleKeyboardInput() in the same way.