How to send message in whatsapp without adding contact

WhatsApp gives an official method to chat with an unsaved number. You can start a chat with any number by logging to the web address https://wa.me/phonenumber. Change the last part with the full number, with the country code

For example, if you need to talk with the number +91-93xxxxxx00 in India, you logging to the URL: https://wa.me/9193xxxxxx00.

It automatically opens WhatsApp and you can easily send messages to that number