How to whatsapp a person that is not in your contact list?

1. Open your preferred browser on your smartphone. 
2.  Type the following link in the search bar: https://api.whatsapp.com/send?phone=XXXXXXXXXXX (In place of the Xs type the phone number of the person you want to contact, including the country code, but without the + sign.) 
3.  That means that if the person has an American number (with the +1 prefix), it would look something like this: https://api.whatsapp.com/send?phone=19175550123 
4.  Press 'enter' on your smartphone. A WhatsApp window will open asking if you want to send a message to that phone number. 
5. Press on 'send message'. You will automatically be redirected to WhatsApp with the 'start chatting' window to the person you entered in your phone.