My steps to creating this program:
1. An interface to schedule the next date and time for program as well message you want to broad cast your users. It will also have option to repeat it on daily weekly basis till a time you provide.
2. As soon as the time is submit, static template based on whatapp format will be created.
3. Program will automatically run every hour on server and check if anything is scheduled for that time.
4. If there is any, then it will pick the static template and start sending requests to whatapp server.
I am assuming, app server and mysql will be provided by you.