On Android and iOS, you can enable or disable the native on-screen keyboard for a webview by clicking the "Native On-Screen Keyboard" checkbox on its WebViewPrefab or CanvasWebViewPrefab (or by setting the NativeOnScreenKeyboardEnabled property programmatically):
The following 3D WebView packages support the "Native On-Screen Keyboard" option:
The "Native On-Screen Keyboard" option is not currently supported by 3D WebView for Android with Gecko Engine.
See also: How does keyboard input work?