I want to send an automatic message to a user that completes a webform. As I can't do it from Whatsapp App nor Whatsapp Business App, I had to suscribe to Whatsapp Business Platform (and add a number that can't be used in Whatsapp/Whatsapp Business App). Now I can send message through the API and receive the reply in a webhook. However, I want to have them on the regular app and reply it just from the phone as usual. Can this be done? Or I need to develop a messaging platform just for this?
Unfortunately you will have to develop your own platform, or go to (and migrate to) ISP partners who have their own developed interfaces. I really hope that Meta will release some own interface solution someday.