Install a Kannada keyboard app or enable Kannada input on your phone
On Android, go to Settings > System > Languages & input > On-screen keyboard > Gboard > Languages > Add keyboard > Kannada
On iPhone, go to Settings > General > Keyboard > Keyboards > Add New Keyboard > Kannada
Open WhatsApp and tap the chat box
Switch your keyboard to Kannada
Type your message in Kannada
Send the message
If your keyboard supports transliteration, type Kannada words in English letters and select the Kannada suggestions