To send a message through the WhatsApp platform, we usually have to have the person added to our contacts, which sometimes can be difficult or complicated but it is not necessary to always do this procedure.
Doing this trick is very simple for Android and iOS devices, it is also free of bans because you do not need to use third-party apps.
The first thing to do is copy this address: https://api.whatsapp.com/send?phone=XXXXXXXXXXX
Instead of the X's, the number of the person with whom we want to contact must be written, with the country prefix forward and without the + symbol. For example, if the number is from Peru and the phone number is 999999999, this would be the link: https://api.whatsapp.com/send?phone=51999999999.
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SOURCE: http://globovision.com/article/conoce-como-enviar-un-mensaje-whatsapp-sin-agregar-a-un-contacto