Install a Kannada keyboard 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
Tap the message box
Switch to the Kannada keyboard
Type your message in Kannada
Send the message
If needed, use transliteration by typing Kannada words in English letters and selecting the Kannada suggestions