WhatsApp Inc. doesn't allow sending messages programmatically for personal accounts. But WhatsApp has started taking requests for business accounts in which they might allow certain programming features like integrating with your own code for sending messages etc.
I had once written a program using selenium to send message via WhatsApp Web. It worked perfectly fine. This was for experimental purpose only.
There are few 3rd-party libraries available but you run the risk of getting your number blocked. WhatsApp tries to detect if you are running such libraries. And if they catch then they will immediately block the mobile number.
Selenium method doesn't have such risk. Because you will be using WhatsApp Web only (via selenium).
For your application, the method suggested by @C-Sway is good enough.