there are already a few Whatsapp solutions around…
How to start Whatsapp/send a message with Whatsapp
CallLog and WhatsApp Utils Extension by Andres Daniel
Taps Swipe Loopup Whatsapp Extension by Andres Daniel
Another Whatsapp Extension by Juan Antonio
Whatsapp Send Message Extension by Ivan Moreno (2 USD)
and the extensions directory is here App Inventor Extensions | Pura Vida Apps
Taifun