Install a Malayalam keyboard app or enable Malayalam input on your phone
On Android, go to Settings > System > Languages & input > On-screen keyboard > Manage keyboards
Enable a Malayalam keyboard such as Gboard Malayalam or another trusted keyboard
On iPhone, go to Settings > General > Keyboard > Keyboards > Add New Keyboard
Select Malayalam if available
Open WhatsApp and tap the message box
Switch to the Malayalam keyboard using the globe or keyboard switch key
Type Malayalam using the keyboard
If needed, use transliteration by typing English letters that convert to Malayalam
Send the message after typing the text