Navigate to the Meta App that is set-up for WhatsApp by going to developers.facebook.com > My Apps > Select your App.
If your phone number is currently registered with WhatsApp Messenger or the WhatsApp Business App, you need to first delete it. See Migrate an Existing WhatsApp Number to a Business Account.
If your phone number fails to register, you will see a message below the drop down that says "Please register your phone number to send messages." Click on the link to access the configuration screen, then click on the slider button next to Registered to register your phone number again.