Whatsapp provides and API for the same which you can use within your Mendix application, refer the below link
You can try using the twilio whatsapp api in mendix! But you need to implement your own logics to send the messages to the whatsapp device using twilio api. refer the below links,
Did it work out with the direct WhatsApp API? The policy of WhatsApp is to offer business accounts via Business Solution Providers like CM.com. We can get your WhatsApp business account up and running in 48 hours, and our API is easy to integrate. Let me know if you need any help.
Jan-Willem van Vugt