A mobile website can be customized to allow users to share a pre-filled message in WhatsApp to a manually chosen contact. As given here it is done using Custom URL Scheme. A
Official WhatsApp doc Says-:
https://api.whatsapp.com/send?phone=countrycode+phonenumber&text=urlencodedtext
Use: https://api.whatsapp.com/send?phone=15551234567&text=urlencodedtext
Don't use: https://api.whatsapp.com/send?phone=+001-(555)1234567
https://faq.whatsapp.com/en/general/26000030