How do we apply for the WhatsApp Business API?

How do we apply for the WhatsApp Business API?

Before you start applying for a WhatsApp Business API, you will need the following: 
  1.   A Facebook Business Manager Account
  2.   A valid phone number for WhatsApp 
  3.   Be subscribed to our reserved plan or a WhatsApp channel in Billing
After you have subscribed to our reserved plan or a WhatsApp channel, go to Channels and start your WABA setup.
1. Go to Channels
2. Click "Start WABA Setup" 

Please refer here for the step-by-step WABA setup process.