Sadly, there is no Zapier integration for WhatsApp. They have a page on the Zapier app directory, but it's currently marked as "Upcoming" and has been for a long time. These days, Zapier lets app developers themselves build and maintain integrations, so it's up to the WhatsApp team to decide if they want to do this.
If you want to send WhatsApp messages using Zapier, your only real option is to make use of Twilio's WhatsApp API. Twilio is a messaging platform, primarily for sending and receiving calls and SMS messages - but more recently they've added a feature that allows you to send WhatsApp messages at a low cost.
Zapier has a Twilio integration, but unfortunately, it doesn't support sending WhatsApp messages. For that, you'd need to use a webhook POST call. We found this guide that can help.