External links do not work in CustomWebView.
The site has a Whatsapp button.
How to make whatsapp open when clicking on the button?
It’s better to ask questions related to CustomWebView extension on the AppInventor forum as the developer can’t answer here:
If you wanted to open the whats app then use. activity starter
Data package: ~WHATS APP PACKAGE NAME~
If you wanted to open the chat for specific number then use
Data Uri: Share on WhatsApp number~
For more info see this article
Using the Activity Starter Component
and before posting search on community.