Where can I refer to develop a chatbot for Whatsapp?

 

 

 This is sample chatbot from caltext whatsapp link. I want to develop an application that can handle host number for whatsapp so I can send message, read input from user and send next message which same as image above chatbot. Is it available on Whatsapp API? I cannot find the document about sending, read user reply and send new message regarding what user reply.