Hi There,
You'll either need to use the WhatsApp Business API for this, or another 3rd party service like Twilio to route the messages through your WhatsApp account. Both of these options are paid-for services, but can be configured to do what you're describing from Filemaker.
There's about half a day's work involved, once you've setup your WhatsApp or Twilio account.
Let me know if you need more clarification,
James.