Open WhatsApp
Open the chat where you want to type
Install and enable a Marathi keyboard on your phone
On Android, go to Settings > System > Languages & input > On-screen keyboard > Manage keyboards
On iPhone, go to Settings > General > Keyboard > Keyboards > Add New Keyboard > Marathi
Switch to the Marathi keyboard while typing in WhatsApp
Type your message in Marathi
Send the message
If Marathi keyboard is not available, use Google Indic Keyboard or Gboard and select Marathi input
You can also type in English and use transliteration if your keyboard supports it