WhatsApp and Viber are 2 different services. If you want to send WhatsApp messages, you need to integrated with WhatsApp. If you want to send Viber messages, you need to integrated with Viber.
From what I see, for Viber you need to purchase to send business messages, that is, you are sending to someone who has not already subscribed. It says right at the top of the API page you linked that if you want to do that, you need to contact one of the official partners. In this case, the API you linked may not be the correct API for using that service.