The new Google Now update lets you send messages to WhatsApp, Viber, Telegram, NextPlus, WeChat (and more promised to be added) using your voice right from the Google App.
To send a message via WhatsApp using Google Now and your voice, do as follows:
1. Make sure both the Google app and WhatsApp are running the latest version
2. Open the Google Now by saying the "OK Google" phrase (or any other method)
3. Now use this command structure: Send a WhatsApp message to [Contact's Name] Saying [Your Message] (For example, Send a WhatsApp message to Steve saying Wanna grab a bite?)
4. And Your Message is sent via WhatsApp to your Contact
** Note that, you may need to confirm the message, as well as the contact's name (in case of similar contacts).
** You can also first say this command: "Send a WhatsApp message to Steve" and then on the opened window, say your message.