The Tracker Program notifications have an option to select ‘webhook’ as the receiving end of the notification.
For your use case you will need to configure an external webhook which will receive the DHIS2 notifications, and then this webhook should be able to make an API call based on the notification content (message and phone number) to the WhatsApp API.
There are official and popular services that might help with making this process easy to setup with an extra cost; however, I’m pretty sure you will need to configure your own secure webhook.
Do you think this is more useful that receiving SMS messages?