Hey Palanivelrajan!
Maybe you can trigger a flow that uses the whatsapp 2018 api as you see on this question:
https://stackoverflow.com/questions/38422300/how-to-open-whatsapp-using-an-intent-in-your-android-apphttps://stackoverflow.com/questions/38422300/how-to-open-whatsapp-using-an-intent-in-your-android-app.
On flow you should have an open url that uses the 2018 api and opens this url:
https://api.whatsapp.com/send?phone="+number
If this doesn't work for you maybe you can find any other method on the whatsapp api, here you have the link of the documentation:
https://developers.facebook.com/docs/whatsapp/on-premises/reference/messages/https://developers.facebook.com/docs/whatsapp/on-premises/reference/messages/
Hope this helps you
Kind regards,
Emilio