Step 1- Go to Settings and click on Whatsapp API Number.

Step 2- Click on Add API Number.

Step 3- Click on Continue.

Step 4-  1. Click on add new API Number
                 2. Select existing API if you've Whatsapp Business API.

Step 5-  Enter the API Number. Note: It should not have Whatsapp Account.

Step 6- Then proceed, Fill your Business Info & Whatsapp Business Profile Info.

Step 7- Give Facebook access to to complete the process and attach the screenshot.