Complete methods from https://graph.facebook.com REST API.
They include /{Version}/{PhoneNumberId}/messages in 2 versions, template and single message.
It includes also the configuration of the webhook to receive messages from customers.
To make it work, the API needs parameters from your WhatsApp Business account.
More details about the usage of WhatsApp in OutSystems in this article:
https://medium.com/@josepedrolimagoncalves/integrating-whatsapp-with-outsystems-d67929651572